In literary analysis, which term best captures a character who serves as a contrast to another character?

The term that best captures a character who serves as a contrast to another character is "foil." A foil is specifically designed to highlight particular qualities or traits of another character, usually the protagonist. By juxtaposing a foil against the main character, the author can emphasize differences that reveal important insights about both characters. For example, if the main character is serious and introspective, a foil could be lighthearted and carefree, thereby accentuating the main character’s seriousness.

In contrast, while the protagonist refers to the main character in a story, and the antagonist represents opposing forces, these terms do not specifically denote contrasting characteristics aimed at highlighting another's traits. An ally usually refers to a supportive character, which does not inherently imply contrast. Thus, "foil" is the precise term in literary analysis that denotes a character whose opposition enriches our understanding of another character's attributes.