Before the Crash
Cassidy was a recently divorced woman whom Sawyer conned out of a known $600,000. After she thwarted his usual routine of "accidentally" dropping the briefcase full of money, she asked him to teach her how to con and they spent the next few months working small scams, while apparently becoming closer as a couple. Once she felt confident in her skills, she asked Sawyer to teach her a long con and offered her own money as capital. Sawyer told his partner that he could not go through with taking Cassidy's money because he had genuinely fallen in love with her. However, his partner replied that he would shoot Sawyer and Cassidy if the con did not go ahead, and in the end, Sawyer finished the con, took Cassidy's money, and disappeared. ("The Long Con")
After her relationship with Sawyer ended, Cassidy set out on the road. While in Iowa, she tried to con people using the techniques she had been taught by Sawyer. Unfortunately, she was not as successful as she hoped. When a man she was trying to con at a service station threatened to call the police, a young lady stepped forward and pretended to buy her fake jewelry to help her out. The woman turned out to be Kate. The two got together for drinks at a local bar where Cassidy promised to help Kate talk to her mother, Diane.
Cassidy first helped Kate by going to Diane's house disguised as a Bible saleswoman who could easily be mistaken for Kate, so that they could have an idea of how heavily guarded Diane's house was. After being taken in for questioning and eventually released, Cassidy and Kate decided to take a different approach. While in the diner that Diane worked at, Cassidy spilled some food on Diane, forcing her to go to the bathroom where Kate was waiting to talk to her.
Cassidy told Kate about her affair with Sawyer but did not mention his name. She told Kate that she was now pregnant with Sawyer's child. Cassidy said she was considering pressing charges against Sawyer but wasn't sure whether to do so. (She did not explain how she planned to find him.) She and Kate then went their separate ways. ("Left Behind")
At some point after that, Cassidy did in fact press charges against Sawyer, and he was convicted of crimes and sent to prison.
While Sawyer was in prison, Cassidy visited him and showed him a picture of a baby girl named Clementine Phillips, whom she claimed was his daughter. Sawyer rejected the notion that he could be her father but later opened a bank account in Albuquerque, in Clementine's name, with the earnings from his prison con. ("Every Man for Himself")
After the Oceanic Six's Return
After Kate returned from the Island, she sought out Cassidy to tell her that Sawyer had been on the Island as well and that he had remained behind. She then gave Cassidy an envelope full of cash for Clementine, on behalf of Sawyer. Cassidy was shocked to hear about what had happened on the Island, but she also told Kate about why she thought Sawyer had stayed behind; because he couldn't face a future with Kate. Cassidy became Kate's confidant, and the two discussed Kate's relationship with Aaron and Cassidy came to the conclusion that Kate took Aaron, not to protect him, but because she "needed him", as how else could she mend her broken heart in the wake of Sawyer leaving her? Kate neither confirmed nor denied Cassidy's assertion. ("Whatever Happened, Happened")
- Kim Dickens, the actress portraying Cassidy, previously starred in the movie Zero Effect in which she played a woman involved in a plan to con her father out of money because she had discovered he had killed her mother because of her pregnancy.
- Of the main characters, she has only met Sawyer and Kate.
- She appeared in flashbacks in two Kate-centric episodes and two Sawyer-centric episodes.