Cassie and Rex were originally known as the Gemini twins, and landed in Salem in the meteor shower of July 2002. They were naked, couldn't speak, and were believed to be aliens. They were taken in by Shawn and Belle Brady, who delighted in teaching them to speak, and in teaching them about humanity. When they twins were taken by Agent Spector for testing, it was discovered that they were indeed human, and the results of genetic engineering, and also discovered that each bore a tattoo of half the crest of the Phoenix (the DiMera family mark), confirming the idea that they were somehow connected to Stefano. After it was discovered that Marlena had given birth to them and that Cassie shared the same rare blood type as Tony, the twins were determined to belong to Marlena and Tony. Cassie was less concerned with her paternity than she was with her interest in Shawn Brady. More than anything she wanted Shawn for herself, and she was willing to do anything to get him, even gas-light Belle Black. Her behavior started getting strange, especially once she and Rex discovered that Roman Brady, and not Tony DiMera, was their father. She was about to sleep with Lucas Roberts when Rex burst into the room with the second shocker ... Kate Roberts was their mother! This meant that Shawn, Lucas, and Philip were all off-limits, as they were all now related.
After learning the final truth about her parentage, Cassie began to curb her wild ways. She made friends with Philip Kiriakis after he was revealed as the mysterious masked dater, and was determined to just be herself. When the murders began in Salem and Rex was arrested, she was desperate to prove his innocence, even if it meant that she would have to commit another murder to clear him, but when push came to shove, Cassie just couldn't do it. The truth about her and Rex's paternity was revealed by Sami at Caroline's funeral, but before Cassie could get the chance to know either of her biological parents as her parents, she discovered the true identity of the Salem Stalker. Unfortunately, the Stalker also knew that Cassie knew, and shortly after sending a message that she knew who the killer was, Cassie's body was discovered stabbed numerous times and stuffed into a piñata at Thanksgiving. Kate was arrested shortly thereafter, having been found with a bloody knife in her hand.
Months later, Cassie was revealed to be alive and well and living on Melaswen. During her time there she had begun forming a relationship with Roman, whom she had only learned was her father days before her death. She was seldom seen, though, and suspected of being in cahoots with Tony, whose name she continued to bear. After it was learned that Tony was the mastermind behind everything, she began referring to herself as Cassie Brady once she appeared in the European castle. Despite her claims of innocence, Marlena often suspected Cassie's motivation. Cassie was able to escape from the castle with the others and return to Salem, but left town with Rex after he broke up with Mimi.