Following the night Marvin Acme is murdered, Santino visits Eddie at his office and tells him of what happened. Later, they both visit the crime scene at the Acme warehouse, where they meet with Judge Doom. Santino informs Eddie of Doom's background of when he bought the election to become the presiding jurist of Toontown. They both also witness a demonstration of the Dip's effect on a Toon, as Doom maliciously melts an innocent Toon shoe.
Santino isn't seen until the after the climax of the film. At the warehouse, he and the police learn it was really Doom, who not only murdered Acme but R.K. Maroon and Teddy Valiant as well. He's then given the rope to the safe that was dropped on Acme as Eddie speculates that the dried paint found on it matches that of Roger's glove. With all of this, Roger is cleared of the murder charge and his innocence is proven.
Hardly anything is known of his history with Eddie, but it can be presumed they've probably known each other during their respective tenures in the police force, prior to the events of the film. Though he carries a stiff demeanor, he looks out for Eddie as he's concerned of his alcoholism and disapproves of his improper conduct toward any superiors.