A is for Antagonist. The definition of antagonist is opponent, rival, enemy, competitor. Someone that shows opposition.
The antagonist is the adversary or opponent to the protagonist. He, she, or they can pose a threat by just being present.
Ex: In The Harry Potter Series, Harry Potter's main antagonist was The One Who Should Not Be Mentioned, The Dark Lord, Voldemort.
Ex: In The Twilight Series, Bella's antagonists were The Volturi in Italy; also James, Laurent, and Victoria (vampires) that were out to destroy her. Or, you could say the Volturi were the antagonists to the Cullens and to Bella.
Ex: Tybalt is the main antagonist in Romeo and Juliet.
Thus, any person or group of people that are out to destroy the protagonist, the main character, is the antagonist.
The antagonist is an integral part of all stories and should be introduced early on in your novel, novella, or short story.
Good always versus evil in most stories. There has to be opposition in all things in order to have a story worth telling.