Cassidy, known as "Beaver" (a nickname he despises), is a son of one of Neptune, California's richest men, Richard Casablancas, and younger brother of town bully Dick Casablancas. Cassidy hangs around with his older brother a lot, even though Dick victimizes him almost as much as his usual targets. Despite his unspoken reservations, Cassidy acts as accomplice when Dick and his friend Logan Echolls cause trouble in the community.
Cassidy is extremely needy as a result of the emotional neglect he suffers at the hands of his father, who openly favors Dick over him. In season three, a guilt-ridden Dick reveals that he and his father would have contests to see which one could make Cassidy cry. Dick also admits to duct-taping Cassidy's feet to the pedals of his bike, forgetting about it and a couple of hours later found him pedaling around desperately in circles.
Some critics have interpreted Cassidy's characterization, especially in the second season, as indicating that the character was in fact a closeted homosexual.
Dick and Cassidy provide an alibi for Logan even though he had returned to Neptune the day of Lilly Kane's murder. Eventually, Cassidy tells Veronica Mars that they had lied to the police.
During Veronica's investigation of her rape at Shelley Pomroy's party, she discovers Dick had encouraged Cassidy to rape her while she was unconscious to lose his virginity. Cassidy, however, denies it. He tells Veronica that he ran out of the room and threw up, after realizing that he could not go through with it.
When his father marries a younger woman, Kendall Casablancas, Cassidy tells Veronica that he suspects she is having an affair and hires Veronica to find proof. In the process, Veronica exposes Cassidy's father's real estate scam. Cassidy's father quickly abandons his family and flees the country.
After a failed request to move back in with his birth mother, Cassidy and Kendall start a new real estate business to make money. With help from Cindy "Mac" Mackenzie as his tech girl, he creates a company named Phoenix Land Trust, using Kendall as the on-paper "owner" of the company, as Cassidy is underage. The Trust buys up cheap properties outside the proposed borders for the City of Neptune, a risky move that bet heavily against passage of incorporation.
Cassidy and Mac soon begin dating, but their relationship falls apart when he refuses to become sexually intimate with her. When she mentions to Cassidy that she had gone to Veronica for advice, Cassidy abruptly dumps her for bringing Veronica into his personal life.
In the second season finale, "Not Pictured", it is revealed that he was among the boys molested by Woody Goodman. Veronica discovers that not only did Cassidy set up the bus crash to kill Woody's other victims, who intended to come forward, but he is also responsible for many of the events of the first two seasons such as orchestrating the exposure of his father's real estate fraud, and Goodman's fall from public favor. She also discovers that he did rape her after all, giving her chlamydia in the process, which he had, in turn, contracted after being molested by Woody. After Cassidy blows up the plane bringing Goodman back to Neptune following his arrest, he tries to kill Veronica. Logan stops him, however, and Cassidy prepares to commit suicide by jumping off the roof of the Neptune Grand. Logan tries to stop him calling him Beaver, to which he replies "My name is Cassidy!" Logan tells him to stop and Cassidy asks why. Logan is unable to answer and Cassidy jumps.