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William ‘Billy’ Costigan Jr
Played By: Leonardo DiCaprio
"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."
A South Boston Police Officer who is selected to become a mole in Frank Costello's organisation.
- Anti-Hero: He has to do some really shady things while undercover, such as participating in murder.
Staff Sergeant Colin Sullivan
A member of Frank Costello's criminal organisation sent to infiltrate the Boston Police Department.
Captain Oliver Charles Queenan
The Captain of the Boston Police Department.
- 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: To keep Costigan's cover secure
Staff Sergeant Sean Dignam
Played By: James Badge Dale
Francis "Frank" Costello
A 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.
- 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.
- 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.
Mr. Arnold "French"
Costello's most trusted enforcer.
Played By: David O'Hara
A slightly dim but intimidating henchman of Costello.
Played By: Mark Rolston
One 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: 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.
- Villainous Friendship: He and Fitzy are friends.
Dr Madolyn Madden
A psychiatrist who Costigan sees under the terms of his probation.
Played By: Kevin Corrigan