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 (this is different
because .