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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.
A foil usually either differs drastically or is extremely similar but with a key difference setting them apart. The concept of a foil is also more widely applied to any 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 Antoine and Pauline Metze explain as: those that emphasize the heightened contrast (this is different because ...), those that operate by exclusion (this is not X because...), and those that assign blame ("due to the slow decision-making procedures of government...").
In Frankenstein, by Mary Shelley, the two main characters of Dr. Frankenstein and his "Adam of your Labours," his "creature, " his "wretch," are both together literary foils, functioning to compare one to the other.
In the Harry Potter series, Draco Malfoy can be seen as a foil to the Harry Potter character; Professor Snape enables both characters "to experience the essential adventures of self-determination" but they make different choices; Harry chooses to oppose Lord Voldemort and the Death Eaters, whereas Draco eventually joins them.