“An antagonist (from Greek ἀνταγωνιστής - antagonistes, "opponent, competitor, rival") is a character, group of characters, or an 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 story where in the action consists of a hero fighting a villain, the two can be regarded as protagonist and antagonist, respectively. The antagonist may also represent a major threat or obstacle to the main character by their very existence, without necessarily deliberately targeting him or her. Examples in both film and theatre include Voldemort, the main antagonist in the Harry Potter series, who constantly battles the series' protagonist, Harry Potter, and Tybalt, an antagonist of Romeo and Juliet, who slays Mercutio and whose later death results in the exilation of the play's protagonist, Romeo.”
—Exact words from Wikipedia.