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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. 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 Pride and Prejudice, Mary's absorption in her studies places her as a foil to her sister Lydia Bennet's lively and distracted nature. Similarly, in Shakespeare's play Julius Caesar, the character Brutus has foils in the two characters Cassius and Mark Antony.
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.
In The Great Gatsby, Nick and Gatsby are often seen as foils to each other.
Another place where foil is used is in the book Tom Sawyer, where the characters Huck and Sid are two prominent foils.
The word foil comes from the old practice of backing gems with foil in order to make them shine more brightly.