A character foil is a literary device used to create contrast between two characters. There are various synonyms for this term. One of them is "contrast character," which refers to the character that is used to highlight the flaws or strengths of another character. Another synonym is "opposing character," which denotes the character who has traits and qualities that are opposite to another character's personality. "Counterpart character" is also a synonym for character foil, which emphasizes the fact that the character serves as a counterpart to the other character in the story. Lastly, "contrastive character" is also commonly used, referring to the character that contrasts the protagonist in a story.