Colin Sullivan

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 secondary, later true main antagonist in Martin Scorsese's crime drama The Departed. He is also the evil counterpart to the film's protagonist, undercover cop Billy Costigan.

Sullivan is portrayed by award-winning American actor Matt Damon.

Biography
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 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. Costigan then 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, a cop who resigned after the murder of his boss, who while a very unpleasant man, sought to avenge the murders of his boss and Costigan. 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.

Trivia

 * After killing Costello, Sullivan became the true main antagonist because he had bigger plans than Costello ever did.
 * He was based on Inspector Lau Kin Ming from Infernal Affairs.