Foil

A foil is a character whose qualities contrast with those of the protagonist (or another major character) in a way that illuminates both. Foils do not need to be antagonists; they can be allies, siblings, or rivals. Their narrative purpose is comparison: by showing what the protagonist is not, the foil makes clearer what the protagonist is.

In Harry Potter, Draco Malfoy serves as a foil to Harry: both are boys entering a world of magic, but where Harry chooses courage and loyalty, Draco defaults to privilege and cruelty. In Sherlock Holmes, Watson's warmth, normalcy, and emotional intelligence foil Holmes's brilliance, coldness, and social dysfunction. Each character is more vivid because of the other.

Effective foils share enough similarities with the protagonist that the comparison feels meaningful. Two characters who have nothing in common cannot serve as foils because there is no basis for contrast. The strongest foil relationships suggest "there but for the grace of God," implying the protagonist could have become the foil under different circumstances.