foil (literature)

In fiction, a foil is a character who contrasts with another character (usually the protagonist) in order to highlight particular qualities of the other character. In some cases, a subplot can be used as a foil to the main plot. This is especially true in the case of metafiction and the "story within a story" motif. The word foil comes from the old practice of backing gems with foil in order to make them shine more brightly.