Sean Dignam was a very unpleasant staff sergeant in the Boston police. He constantly swore and insulted people. Despite his abrasive personality, he was extremely loyal to his boss Captain Oliver Queenan and sought to avenge both Queenan and Billy Costigan's deaths by killing Colin Sullivan, who was Frank Costello's mole in the police.
He comes off very standoffish to anybody he meets but that's because he's only protecting who and what is close to him. Sean has always said this about his job.After Queenan got murdered by Costello's men, DIgnam resigned and took time of to work on the case by himself. At some point near the end of the film Madilyn Madden sent him the boxes of tapes with proof that sergeant Sullivan was the rat all along. He later planned a murder case on Sullivan by waiting for him in his apartment to kill him with a silenced handgun. He then finally avenged his friend and partner.