Antagonist

An antagonist is a character in a story who is presented as the chief foe of the protagonist.
Etymology.
The English word antagonist comes from the Greek ἀνταγωνιστής – "antagonistēs", "opponent, competitor, villain, enemy, rival," which is derived from "anti-" ("against") and "agonizesthai" ("to contend for a prize").
Types.
Heroes and villains.
In the classic style of stories where the action consists of a hero fighting a villain, the two may be regarded as protagonist and antagonist, respectively. However, an antagonist is not always a villain or opponent, just a character provided for the purpose of thwarting or vexing the protagonist. In some narratives, the protagonist is a villain and the antagonist is an opposing hero.
Antagonists are conventionally presented as making moral choices less savory than those of protagonists. This condition is often used by an author to create conflict within a story. This is merely a convention, however. An example in which this is reversed can be seen in the character Macduff from "Macbeth," who is arguably morally correct in his desire to fight the tyrant Macbeth, the protagonist.
Examples from television include J.R. Ewing (Larry Hagman) from "Dallas" and Alexis Colby (Joan Collins) from "Dynasty". Both became breakout characters used as a device to increase their shows' ratings.
Other characters.
Characters may be antagonists without being evil – they may simply be injudicious and unlikeable for the audience. In some stories, such as "The Catcher in the Rye", almost every character other than the protagonist may be an antagonist.
Aspects of the protagonist.
An aspect or trait of the protagonist may be considered an antagonist, such as morality or indecisiveness.
Non-personal.
An antagonist may not always be a person or people. In some cases, an antagonist may be a force, such as a tidal wave that destroys a city; a storm that causes havoc; or even a certain area's conditions that are the root cause of a problem. An antagonist also may or may not create obstacles for the protagonist.
Societal norms or other rules also may be antagonists.
Former protagonists.
A character once a protagonist can turn into an antagonist under extremely negative circumstances. Examples include Anakin Skywalker from the "Star Wars" franchise who turns from a protagonist Jedi to antagonist Sith Darth Vader. A supporting protagonist can become an antagonist by betraying a main protagonist.
Usage.
An antagonist is used as a plot device, to set up conflicts, obstacles, or challenges for the protagonist. Though not every story requires an antagonist, it often is used in plays to increase the level of drama. In tragedies, antagonists are often the cause of the protagonist's main problem, or lead a group of characters against the protagonist; in comedies, they are usually responsible for involving the protagonist in comedic situations.