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In any narrative, a foil is a character who contrasts with another character; typically, a character who contrasts with the protagonist, in order to better highlight or differentiate certain qualities of the protagonist. A foil to the protagonist may also be the antagonist of the plot.
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.
A foil usually either differs dramatically or is an extreme comparison that is made to contrast a difference between two things. Thomas F. Gieryn places these uses of literary foils into three categories, which Tamara A. P. Metze explains as: those that emphasize the heightened contrast , those that operate by exclusion , and those that assign blame.