Antagonist

The antagonist of a story is a character or external force that challenges and/or opposes the goals and values of the protagonist.
Many stories have antagonists in order to create tension, conflict, or relationships. However, not all do, such as narratives that would fall under the slice-of-life genre. 
Antagonists come in a variety of forms, due to the level of threat they pose, the types of challenge they create for the protagonist(s), and the goals and motivations they have. All of these factors define their role and function within the larger story as an antagonist.
== Types ==
Within a narrative, antagonists may challenge the protagonist(s) in a variety of ways, not all of which are equivalent or similar. It is not unusual for a story to have multiple antagonists to serve different purposes. 
Below are common types of antagonists, though not all antagonists will fall into one of the below categories.
=== Rival ===
A rival is a character who is on similar or equal footing with the protagonist and sets out to intentionally create challenges for them, often by means of competition, but not necessarily out of malice, troublemaking, or dislike. Friendly rivals compete with the protagonist in order to get better at what they are competing at, while at the same time encouraging the protagonist to improve as well. They may even consider the protagonist their friend, and vice versa. Many examples of this can be seen within sports anime and newer Pokémon games.
Rivals who are motivated by malice, troublemaking, or dislike usually pose a similar level of low threat and are ultimately still devices to challenge the protagonist in some way. An example of this is Draco Malfoy during the first four Harry Potter books, or Silver from Pokémon Gold/Silver.
Many fans tend to be fond of rivals or rivalrous relationships, often shipping them into the very popular rivals-to-lovers trope, regardless of the level of antagonism coming from the rival.
=== Enemy/Bad Guy ===
An enemy is a type of antagonist who not only opposes the protagonist, but who the protagonist has decided to oppose as well. Oftentimes their existence or relative closeness to the protagonist is itself a challenge, and requires the protagonist to act more direly than they would otherwise. Antagonistic rivals are often also enemies, though an enemy is not inherently a rival. "Enemy" itself is a blanket term that may apply to a variety of level of threats, but generally implies hostility and malintent toward the protagonist.
Similarly, a nemesis is a long-standing enemy who has or whose existence has repeatedly challenged the protagonist over a significant period of time.
Much like rivals, many fans tend to be fond of enemies and ship them with the protagonist into the equally as popular enemies-to-lovers trope. Some fans, however, particularly among anti-shippers, may consider the treatment from one character to another within these enemyships to be bullying, abusive, or so over-the-top that they oppose many types of these ships.
=== Villain ===
A villain is an antagonist who ultimately intends to or is willing to harm others for the sake of their goals. They always pose a larger threat to others, not just one character. Villains can also be protagonists, such as Light from Death Note; they are not inherently antagonists. Villains oppose heroes, not protagonists, and are more defined by their moral compass rather than their role in the story. 
For serialized stories, such as episodic TV shows, it is not unusual to have a "bad guy/villain/monster" of the week - someone destructive who the heroes have to take down lest they hurt others, but are easy enough to defeat within the episode and not be concerned with again.
=== Little Bad ===
A Little Bad, coined by the Spoiler Slayer in reference to Buffy the Vampire Slayer, is a villain who poses a threat for longer than an episode, and may be considered a large long-standing threat for a period of time before an even bigger bad is revealed to be behind them. They are much like level/world bosses in video games: an antagonist who poses a higher challenge than the average enemy and has more control or power over them, but is not the Big Bad in the end. They can be seen as milestones during the hero's journey due to their increase in power and challenge for the protagonist, testing the things that the protagonist has learned and overcome so far.
=== Big Bad ===
A Big Bad, a term also originating from Buffy the Vampire Slayer fandom, is a villain who poses a larger threat than any other previously introduced, and is usually significantly more powerful than them, either on equal footing with the protagonist, if not more. Their presence is a threat a for a much longer period of time, spanning episodes, books, or seasons; and oftentimes throughout a story's overall run. It is also not unusual for a Big Bad to have been plotting for quite some time, understanding that the protagonist is as much of a threat to them as they are to the protagonist, and commanding or employing other characters for the protagonist to face before facing them. They are almost always the Final Boss in stories where the big bad is a villain, and the protagonist is a hero. Examples of this are Ganondorf from the Legend of Zelda, Voldemort from Harry Potter, Bill Cipher from Gravity Falls, and Moriarty from BBC's Sherlock (during seasons 1-2).
Shows like Buffy the Vampire Slayer or Arrowverse may have multiple Big Bads, wherein each season or collection of seasons has its own dedicated large-scale threat that the protagonist(s) must be introduced to and learn how to defeat, but is ultimately no more or less powerful than other Big Bads.
== Resources ==
* Inconsistent Heinous Wiki - Wiki for villains who aren't totally evil, but who exhibit good traits on occasion.
* Fandom responses to the presentation of the ideology of antagonists
* Shall we begin? - a multifandom villain vid by Dominil13
* Evil.Net - "where the bad guys win"; an archive of villain-friendly websites from a variety of fandoms, including Disney.