Colin Sullivan grew up in the Southie Projects. His parents were both dead, he was raised by his grandmother. As a child he was very religious, a strong student, and a good person. When he was twelve years old his path changed. Colin met Frank Costello the Irish Mob Boss, who became his father figure. When Colin grew up he joined the Police Force. Costello asked him to inform for him. He became extremely successful in the force, quickly rising through the ranks, and made his way to the Special Investigations Unit, who were trying to nab Costello. Colin moved into a very nice apartment on Beacon Hill, and got a girlfriend named Madolyn. After a turn of events, the cops figured out that there was a mole in the SIU and hired Colin to find the mole (himself). Colin attempted to set up Queenan, the Police Chief, as a potential mole, in order to get info for Costello. After men followed Queenan, Colin tipped off Costello and made it possible for Costello to kill Queenan. Dignam, the Undercover Unit head, quit his job in fear, knowing that Colin was the mole. Colin found out from Queenan's files that Costello was an FBI informer, and had been giving information to the FBI. Colin turned against Costello, and led the force on an attack on Costello. After a showdown between the two, Colin killed Costello and was praised as a hero. Billy Costigan, the Police informant, came in to resign that night, but he found evidence that led him to believe that Sullivan was the rat he had been looking for, and in a panic, he ran. Sullivan then deleted Billy's information, and the two met up finally to face off. Billy handcuffed Sullivan, and was going to take him in, but another informer for Costello, Barrigan, killed Billy. In a panic, Sullivan killed Barrigan and covered it up well. But soon after, Dignam took revenge, and killed Sullivan for his actions.