In fiction

Fiction

Fiction is the form of any narrative or informative work that deals, in part or in whole, with information or events that are not factual, but rather, imaginary—that is, invented by the author. Although fiction describes a major branch of literary work, it may also refer to theatrical,...

, a foil is a character who contrasts with another character (usually the antagonist

Antagonist

An antagonist is a character, group of characters, or institution, that represents the opposition against which the protagonist must contend...

) in order to highlight particular qualities of another character.

A foil's complementary role may be emphasized by physical characteristics. A foil usually differs drastically. For example in Cervantes

Cervantes

-People:*Alfonso J. Cervantes , mayor of St. Louis, Missouri*Francisco Cervantes de Salazar, 16th-century man of letters*Ignacio Cervantes, Cuban composer*Jorge Cervantes, a world-renowned expert on indoor, outdoor, and greenhouse cannabis cultivation...

' Don Quixote, the dreamy and impractical Quixote is thin in contrast to his companion, the realistic and practical Sancho Panza

Sancho Panza

Sancho Panza is a fictional character in the novel Don Quixote written by Spanish author Don Miguel de Cervantes Saavedra in 1605. Sancho acts as squire to Don Quixote, and provides comments throughout the novel, known as sanchismos, that are a combination of broad humour, ironic Spanish proverbs,...

, who is fat. Another popular fictional character, Sherlock Holmes

Sherlock Holmes

Sherlock Holmes is a fictional detective created by Scottish author and physician Sir Arthur Conan Doyle. The fantastic London-based "consulting detective", Holmes is famous for his astute logical reasoning, his ability to take almost any disguise, and his use of forensic science skills to solve...

, is tall and lean; his right-hand man Doctor Watson, meanwhile, is often described as "middle-sized, strongly built." The "straight man

Double act

A double act, also known as a comedy duo, is a comic pairing in which humor is derived from the uneven relationship between two partners, usually of the same gender, age, ethnic origin and profession, but drastically different personalities or behavior...

" in a comedy duo is a comic foil. While the straight man portrays a reasonable and serious character, the other portrays a funny, dumb, or simply unorthodox one.

Laertes and Fortinbras in Shakespeare's play Hamlet are also examples of character foils- all faced with the dilemma of how/whether to achieve revenge for the murder of their fathers, Laertes and Fortinbras serve to highlight Hamlet's morality
In some cases, a subplot can be used as a foil to the main plot. This is especially true in the case of metafiction

Metafiction

Metafiction, also known as Romantic irony in the context of Romantic works of literature, is a type of fiction that self-consciously addresses the devices of fiction, exposing the fictional illusion...

 and the "story within a story

Story within a story

A story within a story, also rendered story-within-a-story, is a literary device in which one narrative is presented during the action of another narrative. Mise en abyme is the French term for a similar literary device...

" motif.