Antagonist

An antagonist (from Greek ἀνταγωνιστής - antagonistēs, "opponent, competitor, enemy, rival") is a character, group of characters, or institution, that represents the opposition against which the protagonist must contend. In other words, 'A person, or a group of people who oppose the main character, or the main characters.' In the classic style of stories wherein the action consists of a hero fighting a villain/enemy, the two can be regarded as protagonist and antagonist, respectively.