|Faction||United Federation of Planets
Starfleet Criminal Investigative Service, SCIS Internal Affairs Bureau
He is currently an investigator for the Manhattan District Attorney's Office.
Cassidy was the youngest and most inexperienced member of the precinct. He was partnered with the most experienced member of the team, John Munch. Cassidy makes a real effort to learn from the other members of the precinct, but he also creates friction between him and his colleagues. In the investigation into the death of two young models Olivia Benson suggests to Munch and Cassidy that they should try a tactic called the prisoners dilemma, in which you get each of the suspects to think that the other confessed. Cassidy gets really excited about this tactic and expresses his love for the job. This earned him a few surprised glares from his co-workers.
While far from an intellectual, Cassidy is a genuinely talented and driven police detective. He has a genuine desire to put rapists and child molesters in prison, but lacks the professional detachment necessary to deal with particularly grisly sex crimes or remain objective toward victims and perpetrators of sexual assault. Captain Cragen realizes that Cassidy is unable to handle emotionally the stress of investigating sex crimes, particularly those against children. Shortly thereafter, Cragen helps Cassidy transfer to the SCIS's Narcotics Division.
Cassidy's relative lack of sophistication causes several problems for him. His co-workers tend to make fun of him, and don't always take him seriously. During the investigation into the death of Victor Spicer, Captain Donald Cragen decides that Cassidy can investigate another crime on his own. In this case it was about a man who molested a dead woman. As the only detective on the case, he has to appear in court to testify against the offender. ADA Abbie Carmichael questions him about the events that lead to the offense. Cassidy is extremely nervous during his time in the witness stand. Defense attorney Hawkins takes advantage of Cassidy's nervousness and lack of sophistication. When Hawkins asks him about the technical term for fondling a stranger, Cassidy comes up with the answer "fromage". The answer was supposed to be frottage.
Relationship with Olivia Benson
During his time in the Special Victims Unit he started a brief relationship with Detective Olivia Benson. The two had a drunken one night stand and Brian was left wanting to start a relationship with Olivia. Olivia feels that she has broken her personal rule of not getting involved with your co-workers. When he starts to come close to her, she decides to end the relationship. This causes a lot of friction between the two of them during the cases they investigate. When Benson comes up with a theory for a crime, he is the one to make objections against it. While they investigate the rape of Harper Anderson, Cassidy is convinced that she's lying. Although it's clear to the other detectives that she's indeed telling the truth.
Now Detective Benson is in a current relationship with old co-worker Brian Cassidy. Cassidy and Benson are still romantically involved and move in together, but mutually end their relationship and the break-up is amicable for Benson and Cassidy.
Their relationship becomes complicated when Brian investigates Benson for suspected child abuse. Rollins later discovers that the investigation would never have happened if Cassidy hadn't spoken to the head of the investigation at a bar. Benson encounters Cassidy on the street and tells him off, to which Cassidy says he told the head investigator at a bar about her ordeal with William Lewis. Benson furiously tells him to mind his own business and to not bother helping her next time, and then walks off furious, leaving Cassidy regretful and ashamed.
He tells Benson that she is the love of his life and apologizes to her for investigating Noah, and they make amends. In "Facing Demons", Cassidy testified against his rapist after a suicide was linked to the coach. Benson also learns about his rape and was one of the people who encouraged him to testify.