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Character Role Analysis

Billy Costigan/Colin Sullivan

In a way, Billy and Colin are two sides of the same coin. Billy is a rat for the police who will do almost anything to take down Frank Costello. Colin, on the other hand, is a rat for Costello who will do almost anything to make sure the cops never take Frank down. Both guys are extremely good at pretending to be someone they're not. But in the end it's Billy who discovers Colin's identity as Costello's rat. Colin ends up outliving Billy, but not for a very long time. Both men knew what they were getting into when they decided to become rats, and both end up paying the price for it.