What term describes a character whose personality contrasts sharply with those of another?

The term that describes a character whose personality contrasts sharply with those of another is indeed "foil." A foil serves to highlight particular qualities of another character, usually the protagonist, by providing a stark comparison. For instance, if one character is brave and reckless, a foil might be cautious and reserved, emphasizing the traits of the protagonist by juxtaposition.

In literature, this technique is effective in deepening the reader's understanding of the main character's attributes and motivations. The use of a foil can create tension in the narrative and elevate the themes explored by contrasting outlooks, behaviors, or values. Such character pairings allow the audience to appreciate the dynamics of character interactions and the complexity of individual choices.

The other terms refer to different concepts. An antagonist is typically the character that opposes the protagonist, often driving the conflict. A protagonist is the main character around whom the story revolves, not necessarily defined by contrast. A static character is one that does not undergo significant change throughout the story, unrelated to the function of highlighting another character's traits.