Colin Sullivan is a high-ranking yet undercover cop within the Boston Police department who is acting as a mole for nefarious mob boss Frank Costello. He serves as the protagonist villain of Martin Scorsese's crime drama The Departed. He is also the dark counterpart to the film's protagonist, undercover cop Billy Costigan.
Colin Sullivan was about 10 years old when he met mob boss Frank Costello, who soon took Sullivan under his wing.
20 years later, Sullivan graduates from the Boston police school but what they don't know is that Sullivan is working for the Costello as an informant. But little did he know that the police also planted an informant in the mob. He then starts a relationship with Madolyn Madden who Billy Costigan (the undercover cop) is also seeing under terms of his probation. After Costello escapes a sting he realizes that there might be a police mole. Sullivan becomes determined to find to find out the identity of the rat. One night in a porn theater, Sullivan tells Costello to gather his gang's Social Security numbers so he could find the informant.
Later, Costigan and Sullivan see each other and Costigan chases Sullivan. Sullivan hides from him, but then stabs someone because he thought it was him, only to realize he stabbed a homeless man. Sullivan then learns that Costello is also an FBI informant. Sullivan confronts Costello after he foiled a drug shipment pickup by Costello. Costello says he only gave up people who were already going down. Sullivan asked him if the FBI knew about his connection to Costello. Costello denied it, saying he would never give him up, because he was like a son to him. Sullivan inquired if that was all it was about, all the murder, sex and "no sons", suggesting Costello was sterile. This enrages Costello and he retaliates by shooting at Sullivan, who is unharmed due to his body armor and responds by killing him with a few gunshots.
When he gets back to the police station he is hailed as a hero for killing Costello. But looking though files, Costigan learns that Sullivan is the mole all along. Sullivan likewise discovers that Costigan knows his secret and erases all evidence that Costigan worked for the Police to neutralize him as a threat, knowing that the only persons who knew Costigan were Captain Queenan (who had been murdered by Costello's thugs) and Dignam (who resigned after Queenan's death). However, shortly afterwards, Costigan receives a package from Costello's attorney, which was forwarded to him in the event of Costello's death, as Costello ironically deemed Costigan to be trustworthy. The package contains tape recordings proving Sullivan was Costello's mole. Armed with this evidence Costigan sends a message to Sullivan to meet him on the roof of the old mill. There, he arrests Sullivan but as they go down the elevator the doors open and then Costigan is shot in the head by a corrupt cop named Barrigan, who is then shot by Sullivan.
Sullivan then learns that there is another mole in the police, he then kills Barrigan while he's not looking. He goes home after Costigan's funeral, but then he is confronted by Dignam, wearing hospital slippers (to prevent his footstep from being heard) and armed with a suppressed pistol. Dignam had been notified somehow by Costigan that Sulivan was the mole, and he had arrived to kill Sullivan in revenge for Queenan and Costigan's deaths. Sullivan, realizing this was the end, simply says "Okay" before Dignam fatally shot him in the head and left. If Dignam hadn't been there, Sullivan would've been a Karma Houdini.
- He was based on Inspector Lau Kin Ming from Infernal Affairs.
- After killing off Costello, Sullivan took over as the Big Bad for the rest of the film.