An antagonist is someone who contends with, or opposes, another in a fight, struggle, or battle of wills: "Once close friends, they became antagonists in the courtroom." In literature, an antagonist is the principal opponent of the main character involved in a play or novel, a person known as the protagonist. In Greek, protagonist meant "first combatant," the principal or central character. In Shakespeare's Hamlet, the protagonist is Hamlet; among the antagonists are King Claudius and Laertes. See also adversary.
Dictionary of problem words and expressions. Harry Shaw. 1975.