4. Billy Costigan - The Departed
Warner Bros.Of all the characters on this list, Billy Costigan is the only one who isn't really a criminal. By legal terms he is, but in reality he is a mole for the FBI planted in Frank Costello's South Boston outfit in order to bring down the drug kingpin once and for all. His struggles with his own temper and morality render him an extremely three-dimensional character amidst violent sociopaths. The Costigan family is a family that has deep connections to the mob down in Southie, a reputation that William Costigan tries his best to shed when applying to be a police officer. However, those very connections - which cause him constant grief and agitation - are the reason he is chosen to infiltrate Costello's outfit. Billy's role in Frank Costello's gang is an extremely tasking one, requiring this man to betray his ethics time and time again in order to prove to Frank that he is rotten to the core. Despite being an extremely flawed character, Billy Costigan is the most sympathetic one in Scorsese's crime drama. Coping with extreme stress and a volatile temper to match, his internal struggle to balance out his extreme personality with a double life creates great drama - especially when he is hunting down a mole with the exact reverse position as him. His brutal demise is one of the most gut-wrenching moments in the entire film, because of how close he came to getting away triumphantly.