Which term describes the character that opposes the protagonist?

The term that describes the character who opposes the protagonist is "antagonist." An antagonist is typically in conflict with the protagonist, serving as the main opponent in the story, which can create tension and drive the plot forward. This opposition can take various forms, such as a villain or opposing force, and the presence of the antagonist often highlights the qualities and motivations of the protagonist.

In literary terms, a foil is a character who contrasts with another character, typically the protagonist, to highlight particular traits of the main character. While allies support the protagonist, the protagonist is the central character of the narrative, and neither provides the necessary opposition that defines the role of an antagonist. Thus, the antagonist is crucial because they not only create conflict but also help to reveal the championing qualities of the protagonist through their interactions and struggles.