Prince Nikolas Cassadine was raised by his Uncle Stefan after his mother, Laura, left him to return to her husband, Luke Spencer, and after Luke killed Nikolas' father, Stavros. As he grew up, he developed a hatred for his mother and vowed that he would never trust her. However, when he met her in Port Charles in 1996, he wanted to become close to her despite lingering reservations about trusting her. Stefan had taken Nikolas to town to save his half-sister Lulu's life by donating his bone marrow to her.
Luke, who had not known about Nikolas until then, forced Laura to choose between Nikolas and her real family. Torn, Laura ultimately chose to remain with Luke and their children and abandoned Nikolas yet again. Hurt, Nikolas felt no need to remain in Port Charles and asked Stefan to take him back to Greece. However, Stefan threatened to have General Hospital shut down if Laura didn't let the Cassadines remain in Port Charles. She was forced to agree to his terms.
During all of this, Nikolas had developed a close friendship with nurse Bobbie Spencer at General Hospital. Bobbie later married Stefan, and Nikolas and Bobbie became closer. However, their friendship quickly ended after Nikolas discovered that Bobbie had betrayed Stefan.
Nikolas' next friendship was with Katherine Bell, whom he first met when she went to Wyndemere, the Cassadine mansion, looking for location shots for a campaign for her company, Deception Cosmetics. When Laura faked her own death to escape the Cassadine threats, Nikolas ran straight to Katherine. Before she could console him as a friend, he pulled her into a passionate kiss, changing everything. When Katherine was found shot at Wyndemere, Nikolas immediately accused Luke, but in actuality, it had been Stefan who pulled the trigger, thinking that there was an intruder on the island.
When Nikolas discovered that his mother had faked her death, he banished her from his life. He still remained close to his sister and grandmother, though, and decided to stay in Port Charles and attend public school. He then became involved with Sarah Webber, which caused further resentment with his half-brother Lucky, who had a crush on Sarah at the time. One night at a party at Luke's club, Nikolas became the victim of mob violence when he was shot in the throat with a bullet intended for Jason Morgan, and Nikolas suffered a stroke. Jason then performed a tracheotomy and ultimately saved Nikolas' life.
The shooting left Nikolas with Broca's aphasia, and he was unable to speak for a long period of time. During his recovery, Nikolas became depressed and shut Sarah out because he was ashamed to see her. They eventually broke up. One person that Nikolas did respond to, though, was Robin Scorpio. They developed a bond as Robin helped him with his speech therapy. Stefan, however, did not approve of Nikolas' relationship with Robin. Not only was she a member of the Scorpio family, whom the Cassadines had experienced trouble with in the past, but she was also involved with Jason Morgan. Despite this, Robin and Nikolas remained good friends.
Nikolas was happy to learn that Katherine and Stefan were engaged. He liked Katherine as a friend and knew that she would be good for his uncle. On the night of the Bacchanalia, the party celebrating their engagement, Katherine fell to her "death" off a parapet after overhearing a conversation between Stefan and Laura and learning that Stefan was Nikolas' father. Nikolas was devastated, and of course blamed Luke. However, Katherine later returned from the dead and revealed Nikolas' paternity to him following a fight. Furious, he confronted Stefan and Laura and felt betrayed and upset for being lied to for so long. However, he had always thought of Stefan as a father, and after he got over the anger of being lied to, he couldn't have been happier. Not only was he released from the duties of being a prince, he had the one thing that he had always wanted: a family.
When an unknown person was blackmailing Nikolas' friend Emily Quartermaine, Nikolas joined forces with Liz, Emily, and Lucky to catch the blackmailer. When they caught him, they discovered that he was also Liz's rapist. Through this adventure, Nikolas and Lucky realized that they could count on each other, and they began to build a relationship. Nikolas also began to rebuild a relationship with Katherine, this time a more serious one. Neither Laura nor Stefan was happy when they learned of this, and they were more than horrified when Katherine announced she was "pregnant," and she and Nikolas were getting married.
On the night of the wedding, Katherine repeated history, and again fell off the parapet after being pushed by Helena, who thought Katherine would ruin her grandson's life. This time though, she died. The first person Nikolas suspected was Laura because he knew how much she hated Katherine and how disappointed she was in their relationship. He then learned that Katherine had been manipulating him the whole time and not only was she faking the pregnancy, but she was also barren.
The next tragedy in Nikolas' life occurred when he thought his brother Lucky had died in a fire. He and Lucky's girlfriend, Liz, became closer and tried to help each other through their grief. Nikolas was attracted to Liz, but he knew he couldn't act on it because her heart was still with Lucky. Almost a year later, Nikolas discovered the possibility that Lucky could be alive. He and Liz went to New York City, where they eventually found Lucky. Lucky was not the same person though. He said that he didn't love Liz anymore and started urging Nikolas to take her, but of course Nikolas couldn't because Liz was still convinced that Lucky loved her. Several months later, Nikolas, Liz, and Lucky realized that Lucky had been brainwashed by Helena and, after extensive therapy, Lucky realized that he did still love Liz, and they got back together.
It wasn't long before Nikolas' true paternity was discovered: Nikolas was indeed the son of Stavros Cassadine, making Nikolas the Cassadine prince and Stefan his uncle. The deception had been a plan by Helena to gain access to the Cassadine fortune, which was supposed to be inherited by the prince. She had also enjoyed toying with Stefan's emotions in making him believe he was Nikolas' father. Although angered about being plagued by Cassadine lies again, Nikolas didn't really care what relation Stefan was to him because Stefan would always be his father. A few months later, though, Stefan "died," presumably at the hands of Luke Spencer. But in reality, Stefan had faked his death in order to avoid a real one at the hands of Helena. But Nikolas was unaware of Stefan's plan, and continued to mourn his uncle.
Nikolas met his next great love when he teamed up with Lucky and Liz to catch Emily's blackmailer. They caught her trying to take the cash they had set out as bait. Gia Campbell, the blackmailer, agreed to help them catch the guy who had set Emily up if they kept her presence in Port Charles from her brother. Nikolas found himself attracted to the loud, brash, and beautiful Gia. When she lost her job and had nowhere else to go, he let her move in with him rent-free.
Gia was the only one who Nikolas could really talk to after his uncle returned from the dead. Nikolas was enraged and hurt that his uncle had once again lied to him, and so he shut Stefan out of his life for good. The attraction between Gia and Nikolas built until they finally gave in to it. When Gia lost the chance to be the new Face of Deception model for Deception Cosmetics to Liz, Gia was devastated, and this time, Nikolas helped her. Their budding romance began to blossom more.
After everyone realized that Helena was still using mind control on Lucky, Nikolas realized he would do anything to get his brother out from under Helena's control. He secretly partnered with Luke and Elizabeth in an elaborate scheme to protect Lucky and take down Helena. In order to fool Helena and protect Gia at the same time, Nikolas broke it off with Gia and began to function as a "True Cassadine." In an effort to prove his loyalty, he had his grandmother "remove" Lucky's love for Elizabeth from Lucky's memory, claiming to want Elizabeth for himself. In the meantime, Helena had defrosted her son, Stavros, who then wandered around Port Charles, pretending to be Lucien Cane. Gia tried to move on and even dated Lucien a few times, not realizing his true identity. But she couldn't give up on Nikolas and soon stumbled on his plan of deception. So she joined Nikolas against Helena.
Nikolas met his father for the first time in Helena's lab. Stavros never quite trusted his son's loyalty to the Cassadines and pretended to still be frozen when Helena showed Nikolas her lab. Left alone with Stavros, Nikolas revealed his hatred for his father, not realizing that Stavros heard every word he said. Nikolas told Gia about the meeting, and she comforted him. After Nikolas left to talk to his mother, Gia found a photo and realized that Lucien was Stavros. Laura and Nikolas went to Helena's lab and met face-to-face with the very much alive Stavros. Nikolas tried to play on his father's love for him to convince him to let Nikolas, Laura, and Gia go, but Stavros got mad and locked Gia and Nikolas in a room. Ultimately, Luke defeated Stavros, who fell into a bottomless pit, and Helena was sent to jail.
Back together publicly, Nikolas proposed to Gia, who eagerly accepted. Stefan threw them an engagement party at Wydemere, during which Nikolas' recently discovered Aunt Kristina, Alexis' long-lost sister, revealed that she was taking the Cassadine name as her own. Stefan then announced that Nikolas would be running Cassadine Industries because Stefan was leaving for Milan. Uncertain at first, Nikolas finally accepted this huge responsibility, and Nikolas and Gia moved into Wyndemere.
In the months following Stavros' death and Helena's defeat, Nikolas was plagued with dreams and thoughts of becoming like his father. Gia did her best to comfort him and even gave up her modeling career to focus on her relationship with Nikolas. The night following her last modeling shoot, Nikolas, Gia, Elizabeth, and Lucky went to dinner to celebrate. Sarah Webber, Liz's sister and Nikolas' former girlfriend, returned to town, drawing out feelings of jealousy in Gia. Gia left before Nikolas and got in a car accident on the way home. Nikolas then went into protective mode and covered up the fact that Gia was at fault and that she'd been intoxicated at the time. He refused to let her confess her true involvement and paid off any witnesses.
The only possible crimp in Nikolas' cover-up was Elizabeth, who'd been in the car with Gia and didn't remember much about the accident. Nikolas attempted to distract her and prevent her from inadvertently revealing the truth by persuading her and Lucky to move into Wyndemere. But when Elizabeth finally remembered everything, she blasted them all for their deceptions and moved out. Then Gia's brother Taggert hired an undercover cop to gather dirt on Nikolas and the possible cover-up of Gia's fault in the accident. Nikolas caught on and tried to feed Taggert false information. But the whole investigation became pointless when Courtney dropped the whole thing, and they all moved on.
It became obvious that Gia was bored with just being the fiancée of a prince, so Nikolas paid for her to make a comeback on the cover of a big magazine. But when Gia found out that her talents hadn't earned her the cover, she yelled at Nikolas for trying to control her life. They grew further apart when Gia decided to go back to school to become a lawyer and went to work for Alexis in Alexis' law office. However, the final straw was Gia's growing friendship with Zander Smith. Nikolas was extremely jealous and tried to pay Zander to leave town. Gia found out and broke off their engagement.
Nikolas then focused on keeping his mother and his brother safe from Luke, who had recently gone on a destructive rampage in anguish over losing Laura and seemed to chase Laura deeper into mental illness whenever he went to see her. Nikolas made every effort to keep Laura's location a secret from Luke. In an effort to keep Luke distracted and to stop him from potentially hurting Lucky, Nikolas secretly hired a call girl who resembled a younger Laura to keep Luke's attention. When it was revealed that the girl, named Summer -- her real name was actually Laura -- was working with Nikolas, Luke was angry and hurt.
Summer revealed that Nikolas had blackmailed her into the scam by promising that he knew where her long-lost brother was. But Summer's brother turned out to be an ex-boyfriend who had worked for Nikolas and stolen some valuable coins belonging to the Cassadines. Nikolas had hoped that Summer would lead him to her boyfriend and the coins. Summer teamed up with Luke and Lucky to try to find her boyfriend, and Nikolas followed them. They discovered at the end of the trail that the coins were worthless.
When Nikolas returned from his adventure, Emily visited him. She confided in him that she had breast cancer and had resigned herself to die. She was concealing this fact from her boyfriend, Zander. She asked Nikolas to help her convince Zander that Emily and Nikolas were in love so that Zander would leave Emily and wouldn't have to watch her die. The plan eventually worked, although not before Emily and Nikolas were forced to get engaged to be really convincing. This did not make Stefan happy.
Stefan had recently returned to town because the Cassadines were in danger of going bankrupt, since Helena had squandered away much of the Cassadine fortune. Stefan had been forced to take several loans. The creditors wanted their money, but if that couldn't be delivered, they wanted blood instead, preferably Nikolas'. Nikolas tried to reassure his uncle that he would take care of it, but Stefan had an idea of his own. He led Lydia Karenin to town to marry Nikolas. Lydia was set to inherit a fortune if she married Nikolas, otherwise she wouldn't get anything. The fortune in turn would solve the Cassadine's money woes.
Nikolas' engagement to Emily prevented Lydia from marrying Nikolas. So Stefan arranged for Emily to be pushed off of the cliffs at Wyndemere during her engagement party. But heavy fog that night caused the assassin to mistakenly push Summer, who was at the party with Lucky. She died and in an effort to cover his tracks, Stefan tried to frame Luke for Summer's death.
Nikolas forced Emily to attend a breast cancer survivor group meeting, which inspired her to fight her cancer. It also helped her to decide to tell Zander the truth about her cancer, and she reunited with Zander. But all the pretending Nikolas and Emily had done had led to a real love between them. Torn between his love for Emily and his duty to his family, Nikolas decided to be a dutiful Cassadine and marry Lydia. Lucky was appalled by Nikolas' decision, especially since he knew that Nikolas really loved Emily.
Emily arrived in the middle of the ceremony, and Nikolas nearly called it off. However, Emily told him she only wanted to say goodbye to him. A saddened Nikolas married Lydia, washed his hands of Stefan, and jilted his new bride on their wedding night. Soon after the wedding, a second part to the will's clause was revealed: Lydia and Nikolas had to have a child within the first five years of their marriage in order to gain the full inheritance. This would be a problem since Nikolas didn't intend to ever sleep with Lydia, although one night before they were married, he did out of anger and frustration over giving up Emily.
Nikolas, still very much in love with Emily, and stuck close by when her immune system shut down and allowed her to easily contract meningitis. As Nikolas focused on Emily, Lucky and Lydia worked together to prove that Stefan had been trying to kill Emily when Summer was pushed instead. Lydia was able to get a confession from Stefan on tape, and Stefan was arrested. He was appalled when Nikolas refused to help him. Nikolas claimed that his uncle had become a different person when he'd tried to kill Emily.
Emily hovered near death and married Zander as a last gift to him before she died. But a dream in which she kissed and married Nikolas pulled her back from the other side, and she lived. She informed Nikolas that she wanted to honor her marriage vows to Zander, and Nikolas agreed to respect her wishes. However, they were caught on film when they nearly made love one night. When Zander saw the photos, he went after Nikolas. Nikolas was already in the hospital after receiving a beating ordered by Lorenzo Alcazar, who turned out to be Nikolas' major creditor.
Emily managed to stop Zander from beating up Nikolas and enlisted Jason's help in ensuring Nikolas' safety from Lorenzo. Initially, Nikolas plotted with Jason to kill Lorenzo. But Nikolas changed his mind and borrowed money from Jason instead. Stefan was not happy to hear that Nikolas had borrowed money from a gangster, but Nikolas said that Stefan had done the same when he borrowed money from Lorenzo Alcazar. Stefan then tried to kill Lorenzo in order to rub out the debt completely. After hearing about this, Nikolas informed Stefan that he didn't want anything to do with Stefan anymore.
Finally deciding to follow his heart, Nikolas filed for divorce so that he could pursue Emily. Her marriage to Zander was also floundering, and she was by Nikolas' side when he learned of Stefan's death at the hands of Luke. On the outside, Nikolas showed little remorse and even said "good for Luke" when the cops informed him that Luke had escaped custody. But on the inside, he felt quite guilty and saddened by Stefan's death. At Stefan's funeral, Helena repeatedly insulted Stefan and called his life a waste. But Nikolas told her to shut up and then, with heart-felt emotion, expressed his gratitude for Stefan's care throughout his life, despite the desperate choices that Stefan had made near the end of his life. When Luke sneaked to the gravesite after the service, Nikolas said that the Cassadine/Spencer war had died with Stefan.
Nikolas and Zander continued to fight over Emily, who took a step back from both of them. But she finally made her choice following the Halloween costume ball. When Emily ran off after confessing her love to Nikolas, Nikolas followed her to the cottage. He informed her that his divorce was official and took off his wedding band in front of her. Then Nikolas made love to her for the first time.
Emily and Nikolas happily spent time together in the open again, although they had to contend with Zander attempting to take his anger out on them on more than one occasion. Nikolas also discovered that Stefan's death had really been a suicide. Once Emily's divorce was final, Nikolas proposed. They announced their engagement to her family, along with his poor financial status. But while trying to recover his recently sunken freighter, it was discovered that a sunken ship that had once belonged to the Quartermaines of many generations past was underneath it. Supposedly the Courage sank while loaded with treasure.
Emily and Nikolas told the Quartermaines about the treasure and their intention for the families to split the proceeds. However, Tracy had other plans and hired Sam McCall to recover the treasure for her. Nikolas, in turn, hired Sam's father, Cody. A few days later, Cody's body was discovered in the tunnels at Wyndemere. Nikolas was arrested and put in jail. Soon, they figured out that Zander was blackmailing Ric into framing Nikolas for murder. Eventually Ric grew tired of Zander's threats and tossed him in jail after he provoked Nikolas into attacking him. Nikolas was then freed.
The treasure from the ship had disappeared by that time. Emily and Nikolas tracked the treasure to a hiding place where Helena revealed she had stolen the treasure for Nikolas. They got the treasure back from her and prepared to auction it off. However, the night of the auction, a fire broke out in the Port Charles Hotel, and the treasure disappeared. The auction attendees had to be rescued by helicopter from the roof. But when it was time for the last helicopter to leave, there wasn't enough room for everyone. Nikolas bravely gave his spot to Emily, who tearfully watched as the roof crumbled with Nikolas on standing on it.
Although everyone else believed Nikolas had perished in the fire, Emily refused to believe the love of her life was dead. A few days later, she was at Wyndemere, remembering the good times with Nikolas, when he walked through the door, somewhat battered, but alive. Emily filled him in on the fact that Zander had been killed in the basement by the fire. However, Emily, Nikolas, Ric, and Elizabeth all received letters from Zander stating that each person's significant other had killed him prior to the fire.
The group set about trying to figure out who actually killed Zander. Nikolas admitted he had confronted Zander in the basement before the fire and knocked him out. Then he had locked Zander in the basement and called the police. Ric had arrested Nikolas. However, Elizabeth soon confessed to a confrontation with Zander of her own. She was prepared to confess all to the cops, but Ric wouldn't stand for it. So Emily persuaded Jason to tell the cops that Detective Capelli had confessed to killing Zander before Capelli died. The case was closed …but not for long.
Clues began surfacing that Zander wasn't dead at all. When Emily went to visit the cottage she had briefly shared with Zander, he appeared in the doorway. He wanted to make a fresh start with Emily and claimed to have taken Nikolas. He tried to force Emily to sleep with him in order to save Nikolas, but couldn't go through with it, since it wouldn't be out of love. By that time, the cops had the place surrounded.
Nikolas, on the way back from a business trip, heard on the radio about Zander's return from the dead and that he had Emily hostage. Panicked, Nikolas tried to get back to Port Charles in a hurry. But he ran his car off the road instead. After Zander's standoff with the police ended in his death, Emily learned about Nikolas' accident. However, the police couldn't find Nikolas. Emily searched for days, but slowly realized that Nikolas was gone.
In reality, Nikolas had wandered away from the accident site to a nearby cottage where he collapsed on the front porch. The woman who lived there took him in. When Nikolas awoke, she told him her name was Mary and that she was his wife. She nursed him back to health and continually fed him the past that belonged to her real husband, Connor, who had been killed in the war in Iraq. Nikolas tried to swallow what she told him about his past, but things didn't seem to fit quite right.
As Mary befriended Emily and took a job at GH, Nikolas grew restless and ventured out more often. Although he knew about and had seen Emily from afar, he still didn't remember her. Mary attended Nikolas' memorial service as Nikolas listened from outside the church. Later, he took a job painting the church and met Lorenzo Alcazar. Alcazar was puzzled by the fact that Nikolas introduced himself as Connor Bishop. He offered Connor a job working for him. Mary tried to dissuade Nikolas from accepting, but Nikolas was intrigued and agreed to work for Alcazar.
Nikolas also tried to live in the present and look to the future by proposing to Mary. They were planning their wedding when Alcazar had Nikolas break into Jason's apartment. Nikolas got shot, and Alcazar had him patched up by a private doctor. Then Alcazar offered to send "Connor" and Mary on a trip. Nikolas was drawn to a brochure on Mexico and asked to go there. While in Mexico, Mary and Nikolas "renewed" their vows. While Nikolas was on the terrace of their room, Emily spotted him from the adjoining terrace. She confronted him later, only to discover he was suffering from amnesia and believed he was Connor Bishop. Emily was heartbroken but didn't reveal his true identity.
Back in Port Charles, Nikolas underwent hypnotherapy to regain his memory. He had flashes of memory, but nothing that made him remember Emily or his identity. However, he found himself drawn to Emily and gave in to his feelings one night and made love to her. Afterwards, Nikolas got a call that Mary had nearly drowned in the river. Guilt-ridden, he promised to stop trying to remember and told Emily their night together had been a mistake. However, it wasn't long before Nikolas wanted to remember again. Mary suggested that they have a baby to focus on the future. But Nikolas wasn't sure.
Emily found out that Mary was trying to have a baby and confronted her, threatening to tell Nikolas the truth. In the meantime, Alcazar provided Nikolas with a false history, courtesy of Mary, that painted Emily as a former lover who wouldn't let go of him. When Nikolas returned to confront Mary about it, he found she had taken an overdose of sleeping pills. Believing that Emily had already told him the truth, Mary called him Nikolas. Caught, Mary was forced to explain what was going on and tried to justify her actions. But Nikolas stormed out. He confronted Emily over why she hadn't been the one to tell him the truth first. She told him about her fear of driving him away by the shock of the truth because of the way Jason had left the family. However, Nikolas only saw betrayal instead of love in her actions.
Although aware that he was Nikolas Cassadine, he couldn't remember any of his life as Nikolas. He tried to drink away the pain and betrayal he felt. Lucky and Emily tried to convince him that they loved him, but he yelled at them to leave him alone and insisted that he felt nothing for either of them. However, when he saw them together one night, he punched Lucky. Lucky then had him thrown in jail for the night. Helena tried to have Nikolas committed so that he would be declared mentally incompetent and she could control the Cassadine fortune.
Lucky helped Nikolas escape, but Helena found him later. Emily blamed Mary and believed she had paired up with Helena. In the institution, Helena made sure Nikolas was drugged. Helena attempted to brainwash him into becoming a true Cassadine prince. Emily sneaked in as a patient and convinced Nikolas to only pretend to take his medication so that they could later escape. However, Helena found Emily before Emily and Nikolas could and had Emily drugged as well. Nikolas found Emily and got her out of there. Before they left, he remembered everything from his past. They hid in the Quartermaines' boathouse, where Nikolas shared the good news and they renewed their love for each other.
Nikolas and Emily were back together, but Helena was not happy about it. She and Tracy Quartermaine teamed up against the couple and cursed them. At first, Nikolas and Emily did not take Helena's curse seriously, but them Emily's health mysteriously started to fail without any explanation. To break the curse, the couple headed to the Garden of Aphrodite, located on one of the Greek islands. The curse was broken when they kissed in the moonlight. When Nikolas and Emily announced their engagement, Helena attempted to stab Emily while Emily was standing near the cliffs at Wyndemere. Nikolas arrived just in time to see Helena holding a knife to Emily, so he pushed Helena over the cliffs. Although her body was not found, Helena was pronounced dead.
Nikolas was arrested after he confessed to Helena's murder, but Mac let him out of prison briefly to marry Emily. While Nikolas was in prison, Connor Bishop returned to town. Emily had Connor pose as Nikolas to lure Helena out of hiding so the real Nikolas could be released from prison. On the night Helena resurfaced, Connor raped Emily. The plan had worked, Nikolas was released from prison, but the problems were just beginning for Nikolas and Emily. Emily struggled to look at Nikolas or allow him to touch her, because he looked identical to the man who had raped her.
Nikolas found a friend he could talk to in Courtney Matthews. Her marriage to Jasper Jacks had hit a rough patch as well. Courtney could not have children, so she and Jax hired Elizabeth Webber to be their surrogate. Courtney was upset over how Jax looked at and cared after Elizabeth, and Courtney felt like an outsider. Nikolas felt like an outsider, as well, because Emily was able to talk to her brother, Jason Morgan, and his best friend, Sonny Corinthos, but not to him.
Eventually, Nikolas and Courtney began an affair. When Emily found out, she divorced Nikolas. Then Courtney became pregnant. Nikolas and Courtney stayed together even when two paternity tests showed Jax was the father of the child, and they made plans to fight Jax for custody. Helena interrupted Nikolas' happiness once again by kidnapping a pregnant Courtney. Helena believed Courtney could be carrying the Cassadine heir. Courtney escaped, but soon after, she contracted a deadly virus and required an emergency c-section.
Courtney died after the emergency C-section, but Nikolas was reunited with her briefly and they pledged their love to one another. As Courtney was dying, Jax confessed that he had altered the paternity results. Courtney died with the knowledge that Nikolas was her baby's father, but she never got the chance to tell him. Jax, with the help of Carly Corinthos, kept the baby's true paternity a secret. Robin eventually learned the truth and told Nikolas. He immediately changed his son's name to Spencer as a way to honor his siblings.
Nikolas slowly adjusted to fatherhood and hired a nanny, Colleen McHenry. She had been secretly recruited by Helena to keep a close eye on the Cassadine heir, Spencer, and feed Helena information. During this time, Emily and Nikolas began to date again, but Colleen became obsessed with Nikolas and jealous of Emily. Colleen tried to get Nikolas' attention and drugged him one night and tried to seduce him. When that didn't work, she kidnapped Spencer on Christmas Eve. Nikolas and Emily tracked Colleen and Spencer to Colorado, but they were too late. Helena got there first and took off with Spencer to Greece. Emily and Nikolas arrived just as Helena was about to baptize Spencer and change his name.
Reunited with both Spencer and Emily, Nikolas found that life was going well for him, until he began experiencing unexplained rages and blackouts. Regardless, Nikolas moved forward with his plan to propose to Emily, so he threw a traditional bacchanalia, called the Black and White Ball. During the ball, danger ensued as mobster Anthony Zacchara made an unexpected appearance, along with the Text Message Killer. Jason Morgan, Emily's brother, witnessed Nikolas having an episode of rage, so he tied Nikolas to a chair and urged Emily to leave Nikolas to find safety. Emily refused and remained with Nikolas. Nikolas blacked out, but when he awoke, he was free from the chair, but Emily was dead. She had been strangled to death, and Nikolas feared he had killed her.
After Emily's death, Nikolas was in a state of deep despair, not knowing if he had killed her. The only consolation was his visits from Emily. She appeared to him often, but only to him. Through her visits, Emily convinced Nikolas to seek medical treatment. The diagnosis was clear; Nikolas had brain tumors that were causing the hallucinations, rages, and blackouts. They would be fatal if not removed.
Everyone around Nikolas expected him to have the tumors removed immediately, but it meant he would lose his visions of Emily, so he hesitated. Nikolas found someone he could talk to about his visions, nurse Nadine Crowell. Nikolas had to choose: die with the brain tumor and keep his visions of Emily, or have them removed and never see the visions of Emily again. He didn't like either option, so Nikolas made a secret deal with Dr. Ian Devlin to get illegal experimental drugs that would keep him alive without taking away his hallucinations.
As the rages continued, Nadine stuck by Nikolas' side. She had developed a crush on the grieving prince and was happy to have any of his attention. She helped him remember exactly what he had seen the night Emily was murdered. Nikolas remembered seeing the presumed-dead Diego Alcazar, so he and Nadine teamed up with Jason Morgan to find Diego and expose him as the Text Message Killer. On the night Diego was discovered, Nikolas, Damian Spinelli, and Maxie Jones fought with Diego, and he strangled himself when he tried to escape. With a clear conscience, Nikolas was able to say goodbye to Emily and have the tumor removed.
Nikolas missed his visions of Emily, but he focused on his son and the clinic he had built to honor Emily. Even after the tumor was removed, Nadine continued her friendship with Nikolas and volunteered her time at the clinic. They worked together to get the clinic running, and they used the cover of the clinic to help investigate Dr. Ian Devlin, whose reputation as an upstanding doctor was beginning to crumble. Nikolas found out just how dark Dr. Devlin's past was when Claudia Zacchara washed up on the shores of Wyndemere.
Claudia had been stabbed and kicked into the water by Jerry Jacks. Nikolas nursed her back to health. In a feverish state, she confessed her involvement in the shooting of Sonny Corinthos' son, Michael, and she named Dr. Devlin and Jerry Jacks as co-conspirators. Nikolas was unable to turn her in, as she knew about the illegal drugs he had purchased from Dr. Devlin. Dr. Devlin had left town, but Nikolas refused to help Claudia any longer.
Nikolas and Nadine spent lots of time together, and Nadine was hopeful Nikolas would eventually begin to move on from Emily's death to fall in love with her. Although they did date briefly, the relationship remained one-sided. Then during a hospital lockdown, he saw a woman that looked just like Emily, except her makeup, hair, and clothes were different. Nikolas began to worry the tumor had returned, as no one else had seen her. During the crisis, Nikolas was determined to search the hospital for this woman, much to Nadine's dismay. It was a clear reminder that Nikolas' heart still belonged to Emily.
Nikolas was relieved and fascinated to discover the Emily look-alike was real. Her name was Rebecca Shaw, and Nikolas was infatuated with her, as was everyone that had been close to Emily. She was Emily's twin sister. They had been separated at birth. It was no coincidence Rebecca was in Port Charles. She and her lover/accomplice, Ethan Lovett, were after the Cassadine and Quatermaine money. They planned to use everyone's grief over Emily as a way for Rebecca to get her hands on the money.
Rebecca let herself be taken in by the Quartermaines and got a position at General Hospital. Although the plan was for her to seduce Nikolas, Rebecca preferred spending time with Nikolas' brother, Lucky Spencer, instead. Both Elizabeth and Nikolas had a hard time seeing Rebecca and Lucky together. One night at Jake's, Elizabeth and Nikolas had endured enough of seeing the two flirting so they shared a kiss to make the others jealous. They were both surprised by the kiss, as they had been friends for years and had never thought of one another romantically.
With time, Lucky and Elizabeth got back on track, and Rebecca and Nikolas started dating. It had been Rebecca's plan to become close to Nikolas, but she developed real feelings for him and decided to end things with her secret boyfriend, Ethan Lovett. Before she could confess to Nikolas, Lucky found out about her plan and told Nikolas. Nikolas continued to string Rebecca along until he knew she was in love with him. Then he broke up with her. Nikolas was not in love with Rebecca. He still had his forbidden kiss with Elizabeth on his mind.
Elizabeth was engaged to Lucky, but she was thinking about Nikolas in return. In time, they secretly began an affair, which Rebecca found out about and threatened to expose to Lucky. Instead, she left town, and Elizabeth and Nikolas continued their forbidden romance. Eventually, Lucky found out about the affair and ended his engagement to Elizabeth. Nikolas and Elizabeth felt horrible about what they had done to Lucky and were unable to continue seeing one another.
Then Elizabeth found out she was pregnant. Helena found out Nikolas could be the father and got her hands on the paternity results. Although Lucky was listed as the father, Helena arranged for Nikolas to be listed instead. To Helena, it was a form of revenge to have a Spencer raised by a Cassadine. Although things were strained after the hurt Nikolas and Elizabeth had caused Lucky, they learned to get along for their son Aiden.
Nikolas hired Brook Lynn Ashton to work as his personal escort to his Cassadine business functions. He paid Brook Lynn to attend fancy parties and dinners all over the world. Their relationship was all business at first. Then, after spending so much time together, their friendship and mutual attraction made them lovers, much to Elizabeth's dismay. The relationship between Brook Lynn and Nikolas ended shortly after it began when Brook Lynn left town to pursue her music career.
Shortly after Brook Lynn left, Elizabeth's son Jake was hit by a car and passed away. While Elizabeth and Lucky grieved for their son, Elizabeth produced a new paternity test which showed that Aiden was Lucky's son. Nikolas refused to believe that Aiden was not his but he handed Aiden over to Lucky and left town with Spencer. Lulu visited Nikolas in Greece a few weeks later and was surprised at the changes she saw in Nikolas. He believed that Aiden still might be his son but admitted that he followed what he had always done and placed Lucky's needs ahead of his own. Lulu asked him to return to Port Charles but he explained that he needed to pursue a life for himself separate from the Spencer and Cassadine feud.
Nikolas returned to Port Charles in 2013 just after Lulu had been kidnapped. He was reunited with his mother and Luke and started to tell them what he knew about Lulu's disappearance. As he began to speak, he was shot in the chest and unable to share his information. He required surgery and remained unconscious for several days afterwards.
When Nikolas woke after surgery, he immediately feared for Lulu and explained to Alexis and Elizabeth that his father Stavros was alive and after his sister. He went on to explain that Helena had rescued his father after his presumed death and placed him in a cryogenic chamber on Cassadine Island to heal. After several years, he regained his strength. Helena tried to speed his recovery with news about Nikolas and Spencer and even tried to entice Stavros with the news of a new Cassadine heir by falsely naming Nikolas the father of Elizabeth's son Aiden. Stavros was unaffected by Helena's news but found his own motivation when he saw Lulu.
After Nikolas accidentally learned that his father was alive, he pretended to nurse his father back to health in order to gain inside information regarding his plans. Nikolas was shocked when Stavros set his sights on Lulu and insisted that he would have her. Nikolas had come to Port Charles to warn his sister of the impending danger and was distraught to realize that he had arrived too late.
Nikolas was relieved to learn that Lulu had been rescued and that his father had been killed. Nikolas appreciated all the support and care that Elizabeth had provided and he confessed that he still loved her and wished to resume their relationship. They shared a kiss but afterward Elizabeth declared that she did not want to cause Lucky any more pain and she wanted to explore her new relationship with A.J. Quartermaine. Nikolas believed that Elizabeth should be with him and he planned to win her back.