Story structure
Foil
A character who contrasts with another, typically the protagonist, so the contrast highlights qualities in each.
A foil is "a character who contrasts with another character, typically, a character who contrasts with the protagonist." The word comes from "the old practice of backing gems with foil to make them shine more brightly," and Shakespeare points at the practice himself: Prince Hal's bright metal needs a "sullen ground" beneath it to show its shine, in Henry IV, Part 1. Foil and antagonist can be the same person, since "a foil to the protagonist may also be the antagonist of the plot," but the functions differ: opposition blocks the hero, contrast illuminates. Draco Malfoy faces the same choices as Harry Potter and answers them the other way; Tom Riddle shares Harry's blood status, looks and power but not his morality.
The pairing belongs on the relationship map, each shared trait and each difference a tracked edge; in romance, a rival in a love triangle often works as the protagonist's foil.