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William ‘Billy’ Costigan Jr
Played By: Leonardo DiCaprioA South Boston Police Officer who is selected to become a mole in Frank Costello's organisation.
"You sit there with a mass murderer. A mass murderer. Your heart rate is jacked, and your hand... steady. That's one thing I figured out about myself in prison. My hand does not shake... ever."
- Anti-Hero: He has to do some really shady things while undercover, such as participating in murder.
- Boom, Headshot: How he was killed by Barrigan.
- Cultured Badass: He actually went to decent schools and got excellent grades as noted by Dignam and Queenan, he could actually have had a better career but he really wants to be a cop. He also quotes Nathaniel Hawthorne.
- Deadpan Snarker: Another defense mechanism to deal with the stress of being undercover.
- Didn't See That Coming: There's no way he could have anticipated being taken down like that. He called for backup by calling Brown, he mailed the evidence (making copies) and sent it to the police shrink but it takes a Giant Space Flea from Nowhere, a second mole, to take him out with no warning.
- Expy: his undercover legend of being a former State trooper is analogous to the real-life story of Richard Marinick, a former State trooper who joined Whitey Bulger's gang.
- Foil: To Sullivan.
- Good Is Not Nice: He's quite abrasive, contrasting to the usually calm Sullivan.
- Hair-Trigger Temper: He's like this as the result of the stress of being a mole.
- The Hero: As the film's protagonist.
- Heroic BSOD: He's utterly devastated when Queenan dies in front of him.
- Jerk with a Heart of Gold: His jerk behavior is due to the stress of being undercover within the organization of a murderous madman like Costello.
- The Mole: He's a mole for the Boston Police inside Costello's organisation.
- Sanity Slippage: Due to the stresses of being undercover and then by Queenan's death.
- Sixth Ranger Traitor: To Costello.
- Surprisingly Sudden Death: He's killed by a shot to the head instantly and with no warning whatsoever.
- Unperson: When Sullivan erases all records of him being an undercover.
- Troubled, but Cute: According to Madolyn.
Staff Sergeant Colin Sullivan
Played By: Matt DamonA member of Frank Costello's criminal organisation sent to infiltrate the Boston Police Department.
- Asshole Victim:
- Boom, Headshot: Given to him by Dignam. He gives one to the Mole who whacked Costigan as well.
- Bitch in Sheep's Clothing: Even though he's Costello's mole, he keeps a squeaky clean record to ascend and avoid suspicion
- Detective Mole: He's charged with finding the mole in the Boston police department - himself.
- Dirty Cop: He's Costello's mole inside the Massachusetts State Police.
- Disappeared Dad: Costello takes over.
- Evil Genius: For Costello.
- Expy: of FBI Special Agent John Connolly, Whitey Bulger's inside man within law enforcement.
- Foil: To Costigan.
- Hired to Hunt Yourself: Tasked with finding Costello's mole: Himself.
- Jerkass: What he is at heart. Though he's genuinely a decent boyfriend to Madolyn, until she finds out that he's a mole, she doesn't consider not breaking up with him, even hoping to start a family with him with a child that may or may not be his.
- Mole in Charge: When he takes over Queenan's unit.
- Professional Butt-Kisser: How the rest of the cops see him.
- Raised Catholic: Like most Irish-American kids. The Prolonged Prologue has a shot of a young Sullivan serving as an altar boy. note .
- The Starscream: He ends up betraying and killing Costello after finding out Costello's an FBI informant and was ready to expose him to the feds.
- Ungrateful Bastard: To Costello.
- Wicked Cultured: As a young boy he gets a James Joyce reference and later discusses Freud seriously with a shrink. More to the point he wants to study law and apply himself and make it into Harvard, and maybe become legitimate.
- Villainous Breakdown: At the near end when he's caught by Costigan.
Captain Oliver Charles Queenan
Played By: Martin SheenCostigan's handler in the Massachusetts State Police.
- Big Good: Along with Ellerby.
- Captain Smooth and Sergeant Rough: Captain Smooth.
- Cool Old Guy: He's the nicest character in the movie.
- Defiant to the End: He dies without ever ratting Costigan out
- Destination Defenestration
- Face Death with Dignity
- A Father to His Men: He really cares about the men he oversees.
- The Good Captain: Best exemplified in a scene where he is visited by Costigan in the middle of the night to give some important info. Queenan takes him in, even gives him some food and brings him water which shows he genuinely cares for him.
- Heroic Sacrifice: He lets some of Costello's boys pitch him to his death to keep Costigan's cover secure.
- Reasonable Authority Figure
- Team Dad: To the undercovers.
Captain George Ellerby
Played By: Alec Baldwin
Staff Sergeant Sean Dignam
Played By: Mark Wahlberg
- Badass: Exemplified in the final scene.
- Captain Smooth and Sergeant Rough: Sergeant Rough to Queenan's Captain Smooth.
- Chekhov's Gunman: Appears at the end to kill Sullivan.
- Cluster F-Bomb: Stands out in a film full of them.
- Deadpan Snarker: His snarky lines provide most of the tension breakers in the film.
- Deus ex Machina: When it looks like Sullivan is about to get away with his crimes, he appears in the end to kill Sullivan in revenge.
- Good Is Not Nice/Jerk with a Heart of Gold: He is a rude, obnoxious bully who needlessly antagonizes everyone he comes in contact with, but is dedicated to the job and fiercely loyal to Queenan. He saw to it that Queenan's death, as well as Costigan's (who he didn't even like) were avenged.
- Hair-Trigger Temper/Hot-Blooded: He's very quick to resort to physical attacks.
- Jerkass: Few of his co-workers could stand being around him.
- Jerk with a Heart of Gold: For all his rudeness he's quite fond of Queenan
- The Lancer: To Queenan
- Number Two: For Queenan.
- Officer O'Hara: He's a swearer, and he's also played by native Bostonian Mark Wahlberg, who uses his native accent instead of anything else.
- Sincerity Mode: Drops his profane act a few times, for instance telling Costigan early on that he's too smart to be a cop.
- Sir Swears-a-Lot: Everybody swears in the movie, but he himself hardly says a sentence without a swearing word.
Played By: Anthony Anderson
Played By: James Badge Dale
- Boom, Headshot: Courtesy of Colin.
- Chekhov's Gunman: Plays a massive part in shaping up the ending.
- Detective Mole: Just like Colin.
- Dirty Cop: Just like Colin.
- The Mole: Implied to have been placed on the police by Costello to spy on Sullivan.
- The Scapegoat: Sullivan pins all of his crimes on Barrigan after killing him.
- Walking Spoiler: He is spying on Colin on behalf of Costello.
Francis "Frank" Costello
Played By: Jack NicholsonA ruthless and infamous old mobster targeted by the Boston Police Department.
- Bad Boss: He constantly threatens to murder his underlings if they displease him. He kicks it up a notch at one point with Sullivan, threatening to have his girlfriend raped if he doesn't find the mole.
- Big Bad: All these infiltration games are centred around him and his mob.
- Chronic Backstabbing Disorder: Frank is betraying his own men to the FBI, taping Sullivan so he can give him up if the need arises, and screws the Chinese government over on a deal for stolen missile technology.
- Dirty Old Man: In his first scene, he hits on a teenager by asking if she's had her period yet. Later, he makes a pass at a nun.
- Expy: of mobster James "Whitey" Bulger.
- Faux Affably Evil: He comes across as a charismatic guy for at most, a few seconds, before revealing what a depraved psychotic he truly is.
- For the Evulz: The reason he truly does what he does. At his core, Frank is a sadist.Billy: You don't need the money.Costello: I haven't "needed the money" since I took Archie's milk money in the third grade. Tell you the truth, I don't need pussy any more either... but I like it.
- Genre Savvy: Costello is aware that sending The Mole into the police might lead him to become a double agent, so he sends another mole, Barrigan, to keep an eye on Sullivan.
- The Informant: For the FBI.
- Meaningful Name/Shout-Out: the historical Frank Costello, associate of Lucky Luciano and eventual leader of the Genovese crime family.
Mr. Arnold "French"
Played By: Ray WinstoneCostello's most trusted enforcer.
- Badass: He's a scary guy and the most physically capable of Costello's people.
- Beard of Evil: A small goatee.
- Better to Die Than Be Killed: He ends up committing suicide after getting himself trapped inside the car.
- Deadpan Snarker: He's quick with a witty remark.
- The Dragon: To Costello.
- Exact Words: French isn't gonna hurt ya. He's gonna kill you.
- Genius Bruiser: He's powerful and tough, but he's no idiot. He makes plenty of decisions for Costello's organisation.
- Villainous Friendship: Has a genuine one with Frank, it seems.
Played By: David O'HaraA slightly dim but intimidating henchman of Costello.
Played By: Mark RolstonOne of Costello's top thugs.
- Blonde Guys Are Evil: Per Mark Rolston. Except he isn't evil.
- Dumb Muscle: Seemingly, as he's on the same level as Fitzy.
- Hidden Depths: Revealed as he's dying, when he speaks to Costigan and is later revealed to be an undercover cop.
- I Never Said It Was Poison / Dying as Yourself: Delahunt figures out that Costigan is an undercover cop because he is at the place where the other gangsters were congregating to catch the mole, even though Costigan was accidentally given the wrong address.
- The Mole: He's actually an undercover cop.
- Those Two Bad Guys: Delahunt is usually seen with Fitzy.
- Undercover Cop Reveal: He's actually an undercover BPD officer.
- Villainous Friendship: He and Fitzy are friends.
- Wham Line: "Tell me why I didn't tell anybody."
Dr Madolyn Madden
Played By: Vera FarmigaA psychiatrist who Costigan sees under the terms of his probation.
- Alliterative Name: Madolyn Madden.
- Babies Ever After: She winds up pregnant, with both potential fathers dead. Though its strongly implied that Costigan is the real father.
- Love Interest: For Sullivan and Costigan.
- The Shrink: She's a psychiatrist who treats both Sullivan and Costigan.
- The Smurfette Principle: The only major female character.
- Your Cheating Heart: She sleeps with Billy Costigan while in a (admittedly empty) relationship with Sullivan.
Played By: Kevin CorriganBilly Costigan's cousin.