16-year-old Sharon Collins and her single, paraplegic mother Doris lived in Madison, Wisconsin. When Sharon told her high school boyfriend Frank Barritt that she was pregnant, he wanted nothing to do with her or his child. Sharon and her mother decided to give the baby up for adoption because they felt they were unable to raise a baby properly. The baby girl was adopted by Alice Johnson and named Cassidy. Sharon and her mother moved to Genoa City. Alice later abandoned Cassie to pursue a man and left the child in the care of her elderly mother Millie. Cassie loved Millie, but it was very difficult for a woman of her age to keep up with a young child, and Cassie had no friends but her dolls, and nowhere to play but the dirt.
Sharon ended up marrying Nick Newman, son of wealthy industrialist Victor Newman. When it appeared that Sharon and Nick's newborn baby Noah was going to die, Sharon's best friend Grace Turner and her boyfriend Tony Viscardi tracked down Cassie to help Sharon ease the pain of her loss. They convinced Millie to allow them to return Cassie, now 5 years old, to Genoa City and be united with her natural mother. But Noah lived, so Grace decided to keep Cassie until the "time was right" to tell Sharon. Grace fell in love with the engaging child, and kept her true identity to herself for over a year. After Grace finally told Sharon who Cassie really was, Cassie went to live with Sharon, Nick, and Noah. Alice Johnson later re-appeared and hired a lawyer, Michael Baldwin, to fight for the return of "her daughter." Although Alice was about to win custody, Sharon and Nick were able to convince the jobless and flighty Alice that Cassie would be better off with them. Alice gave up her parental rights and Sharon was awarded custody. After a waiting period, Cassie legally became a Newman when Nick adopted her, and lived with her happy family in a converted barn on the Newman Ranch. Her favorite pastime was drawing pictures for her friends and family, riding horses, and baking cookies. The Newman’s' manservant, Miguel, who called her "Mija," was instrumental in raising her.
In 2000, the Newmans had a scare when Cassie tested positive for tuberculosis, but it turned out be inactive and was cured by antibiotics. During this time sweet young Cassie made friends with Larry Warton, and helped to show his softer side. In 2002 her parents separated and nearly divorced after Sharon miscarried their baby girl, but they reconciled and were living as one big happy family again. Later that year, Sharon had an affair and she and Nick separated once again. After much soul-searching Nick decided to give his marriage another chance. Bringing flowers and a present for Sharon, he paused at the window of their home to see Sharon kissing Nick's father Victor. Furious, Nick called off any idea of reconciliation. When confronted, Victor took the blame, although the mixed up Sharon was once again coming on to any man who was nice to her. Sharon decided her life was a mess and left town. Cassie felt betrayed and angry with her parents after Nick brushed her off, more interested in berating Victor than comforting his daughter. Cassie wandered off and fell through the ice on the Newman Ranch pond. A long time later, Nick finally realized she was missing, jumped into the pond, and rescued Cassie. She was CareFlighted to the hospital and was in a coma for days. Cassie recovered, Nikki blamed it all on Sharon, but forgave Victor for their encounter. Sharon returned months later with a new resolve to not be trod on by any Newman again. Nick made up with her after Sharon seduced him, though Nick did not forgive Victor. Cassie became belligerent and mouthy as a result. It took Cassie years to forgive Sharon for abandoning her and to learn to trust and respect her mother again.
Suddenly Cassie's father Frank Barritt showed up in Genoa City, apparently interested in finally meeting his daughter. Cassie wanted to know who he was, but her parents wouldn't tell her. He just as mysteriously disappeared. Months later it was discovered that Frank was murdered by Cameron Kirsten (the man Sharon was with when she disappeared all those months) in an attempt to frame Sharon for his murder.
Cassie turned 14 and began to hang out at Crimson Lights with her 16-year-old friends, Lily Winters and Sierra Hoffman. It wasn't long before she was trying to act their age, and was caught being secretive, skipping school, and accepting a ride home with an older boy unknown to her parents, so Cassie was grounded. She had a crush on 16-year-old Daniel Romalotti, but he and Lily were getting close-much to the distain of their parents who forbade them to see each other. Word was going around school about a secret party to be held in a park. All the older crowd were going, and Cassie just had to go too-especially since Daniel said he would be there. She managed to talk Nick into un-grounding her for the evening so she could "go to a movie" with her long time friend Ali. Sharon didn't go for it, but Nick persuaded her to give in. Cassie ended up at the party and found Daniel drunk on beer he bought from his friend Sam, trying to get over being separated from Lily. The party ended late, and everyone left. Cassie, who had not been drinking, assured her friends she had a way home, then found Daniel passed out in the back seat of his convertible. Deciding he was too drunk to get home, Cassie took his keys and started the car even though she hadn't had a lesson or learner's permit. CRASH! Since both were thrown from the car, and neither remembered a thing, no one realized that Cassie had been driving. As soon as a bruised Daniel was able to leave the hospital, he was arrested and spent the night in jail. His mother Phyllis bailed him out, beside herself with the thought that the son she had lost for so many years could be taken from her again and imprisoned. Back in ICU after brain surgery, Cassie was fighting for her life. There was hope, so long as infection didn't set in. Suddenly, Cassie sat bolt upright-she remembered-she was driving! Daniel was in so much trouble, but she caused the accident. After attempts to tell her mother Sharon or to use the phone failed, she got out of bed, pulled out her IV and, donning her mother's trench coat, staggered out of the hospital, intent on finding and telling Daniel! After several hours of worry by her family and friends, and searching by the police and detective Paul Williams, she somehow found her way to Crimson Lights. Lily and Daniel rushed to her as she was near collapse. As the exhausted Cassie attempted to tell Daniel the truth, all she could get out was "crash." Someone called 9-1-1, Nick arrived shortly before the paramedics, and he accompanied her back to the hospital in the ambulance. Anxiously, her family held vigil while Cassie's fever heightened and Newman-financed specialists arrived too late to help. Victor, Nikki, Victoria, brother Noah, and Grandma Doris who arrived from her home in Eau Claire, sadly said their goodbyes and I love you’s, and left her alone with Nick and Sharon. One last whisper in Nick's ear that it wasn't Daniel's fault, as he chokingly sang her a lullaby while Sharon hugged her, we saw flashbacks of the adorable child Cassie-the light of their (and our) lives-Cassie slipped away.
The memorial service was touching with Grace, Millie, and Alice conspicuously absent. Larry was said to be out of town, and messages were left for him. Memorial speeches were given by Victor, Nikki, and Sharon. Noah placed Cassie's doll Cindy on her casket with a tear in his eye. During Nick's speech, which was more a threat against Daniel, he spotted Phyllis who had snuck in the back, spoke right at her, then marched to her afterward and demanded she leave. Jack ushered her out in embarrassment.
Sharon was handling her loss, mostly just feeling responsible for letting Cassie go out that night and regretful for all the things Cassie missed out on in her life. Meanwhile Nick was angry, refused to accept any responsibility, was out for revenge against Daniel, and had taken a leave of absence from Newman, putting Victor back in charge. The day after the funeral, Larry showed up at Sharon's door with comforting words about how lucky they were to have Cassie in their lives, and reminded Sharon that he held Cassie responsible for changing his life. He took Sharon on an exhilarating motorcycle ride to help cheer her up.
After a cross-country manhunt, fugitive Lily Winters was out on bail for aiding a fugitive, and Daniel went on trial for vehicular manslaughter where Christine and Paul failed to prove that Cassie was driving the car. But in a last-minute save, Nick Newman turned over the evidence he'd been withholding-the clothes Cassie was wearing that fateful night. The boot she was wearing matched a piece of a heel found under the accelerator. The judge determined that Cassie was driving and let Daniel go free. Lily's parents made a deal to send her away to boarding school in New Hampshire to get the DA to drop the charges against her.
Each year on the anniversary of her death, her family and friends in Genoa City memorialize her in one way or another. Her portrait hung on the wall of Crimson Lights for years, anti-drunk-driving messages include "Remember Cassie", her family visits her grave, and even reads from her favorite book, "Dreamland" by Lewis Carroll.
In July 2013, Sharon overheard Nick and Phyllis discussing the fact that Summer's paternity test from just after her birth had been inconclusive, and that Nick had told Phyllis that it showed Summer was his, and not bothering to get another test. But when Summer and Kyle became romantically involved, Nick needed to get another test to find out if they were actually brother and sister. After the test results showed that Nick was not Summer's father, Sharon was seen admitting to Cassie at her grave, that she had done something she shouldn't have. Later, a grown-up Cassie began making appearances to Sharon, which were apparently hallucinations. Sharon joyously admitted that she was waiting for Nick to come back to her after Summer's paternity comes out, and they could return to being a happy family again.
Nick told Sharon about Summer's paternity, who said that she understood, and would always be there for him. Later Sharon admitted to her hallucination of Cassie, that she had altered the paternity test results, and that Summer was actually Nick's.
Days later, Phyllis was horrified to overhear Sharon at Cassie's grave telling Cassie that Nick would hate her if he ever found out that she had changed the paternity test to show that Summer was not his. When Phyllis' cell phone rang, Sharon realized that she had been caught. Phyllis told Sharon that she was going to the MS gala to tell Nick. Sharon followed Phyllis and caught up to her in the stairwell where they stood on the landing arguing. Phyllis yelled, "Get out of my way, you demented bitch," and called Jack. While grappling with Sharon for her phone, Phyllis fell down the flight of stairs. Sharon found a pulse and spilled her purse trying to get to her phone to call for help. Nick and Avery came into stairwell and kissed on the landing while Sharon snuck out the door at the bottom of the stairs. Sharon arrived home and Cassie listened to her story, supporting her for what she had done, as Cassie never would have when she was alive. Following successful surgery, Phyllis lapsed into a coma, and Daniel took her to an island off the coast of Georgia to give her the best chance to recover. Sharon confided in hallucination of Cassie, that although she felt guilty, she was confident that she and Nick would reunite.