Colin Sullivan is a fictional character from the film The Departed, directed by Martin Scorsese. He is portrayed by actor Matt Damon and serves as a mole for the Irish mob within the Massachusetts State Police. Sullivan's character is complex, navigating the challenges of loyalty and deception throughout the story. In the film, Sullivan's role is pivotal as he works to protect the interests of the mob while simultaneously trying to maintain his cover. His actions create tension and conflict, particularly with Billy Costigan, an undercover cop played by Leonardo DiCaprio, who is trying to uncover the mole within the police force.