When Grant grabs a gun, Will is shot trying to get the gun away from him, and Finn is shot trying to pull Will to safety. He stays with Will as he dies, holding his hand and comforting him.
Alicia finds Finn at the hospital, questioning him about Will's last moments, and the voicemail he left.
Finn is an ASA under State's Attorney James "Jimmy" Castro. He goes up against Lockhart-Gardner several times, and most recently is the ASA assigned to the drug case against Cary. The State's Attorney's office plans to use Cary to get to his/ Alicia's client, Chicago druglord Lemond Bishop.
Not too much is known of Finn's personal life. He was married, and it is suggested his wife left him after their miscarriage. It is also mentioned that he and his ex-wife have a young son. His sister died of a drug overdose, and Alicia suspects this may be a factor in his participation in Cary's arrest. Eli Gold finds information that Finn may have once bribed a judge in exchange for a relieved sentence for his sister.
Finn is portrayed as a very effective lawyer- intelligent, charming, and dogged. His kindness and sense of justice is often remarked on by other characters, but he can also be impetuous and stubborn. He dislikes scheming and manipulating, and has a strong, near-antagonistic reaction to Eli Gold.
Finn and Alicia connect over Will's death when she seeks him out for answers. In Season 5 episode 17, "A Material World," he confides in Alicia that he and his wife lost a child in a miscarriage, advising her that grief "doesn't get solved quickly, and you can't just shake it off." He offers his sincere support, and she thanks him.
When The State's Attorney's office attempts to pin the Grant shooting on Finn - claiming he irresponsibly put Grant in general detention to put pressure on him- Alicia steps in as his lawyer. Part of her counsel to protect Finn from termination is for him to run for State's Attorney himself (as Castro firing his new "opponent" looks condemning).
While Peter initially endorses Finn's run, he becomes suspicious of Alicia's relationship with him after Castro brings him a surveillance photo of Finn leaving Alicia's apartment. Despite Alicia's denial, Peter eventually drops his support, and Finn drops out of the race to resume his position as an ASA.
Finn and Alicia appear to have a strong, burgeoning friendship. They admire one another professionally and personally, although Cary's arrest, and Finn's successful motion to remove Alicia as his representation, lays the groundwork for possible tension.
Peter first encounters Finn in "A Material World," when the latter is leaving Alicia's apartment. Finn truthfully tells Peter that Alicia is helping him with a case. Later, when Finn announces his run for state's attorney, Peter endorses him. However, after Castro attempts to threaten him with a photo of Finn leaving Alicia's apartment (on a different occasion that the first), Peter becomes suspicious. Eli asks Alicia if she slept with Finn, which she denies, offended.
Peter eventually refuses to make his endorsement official, and Finn pulls out of the race.
Eli steps in to groom Finn for his campaign at the Alicia's urging. Finn does not take well to Eli, finding him tactless and somewhat underhanded. After his first interview, Finn feels disgusted with himself for answering the reporter's invasive questions, and Eli informs him that's what politicians do. While Eli respects Finn's kindness and upstanding morality, he informs Finn those things don't win an election.
Eli later finds information that Finn may have bribed or pressured a judge to get a lenient sentence for his drug-addicted sister. Despite Finn maintaining it was the right thing to do, Eli believes the scandal wouldn't be "survivable," and that Peter can't endorse Finn.
James "Jimmy" Castro is the current State's Attorney. Finn and Castro go "way back," Finn having even been the best man at Castro's wedding. It is implied Castro brought Finn to Chicago to have an ally and capable lawyer in the office. However, when pressure is put on Castro to answer for the Grant trial and shooting, he is willing to sacrifice Finn to investigation and blame. With Alicia's help, Finn is able to keep his job and reputation.
In the first episode and second episodes of season six, "The Line," and "Trust Issues," Finn and Castro appear to have mended fences, as they discuss and plan the Bishop case together without sign of conflict.