Foil
A dramatic character who enhances or deflates another character by contrast. Based on the literal jeweler’s foil, or black velvet cloth held behind a piece of jewelry to allow it to shine in contrast. Horatio, in Shakespeare’s Hamlet, represents the definitive foil to Hamlet. Foilis a character who is different from another character and the foil character's difference highlights the qualities of the other character, which most of the time is the protaganoist. Another way to interpret a foil character would be to explain them as a sidekickwith a contrasting personality.
Types of Foil Characters
A foil character can come in many different forms. It can be the villain in a story who fights the protagonist and therefore causes conflict for the character. A different type of foil character does not have to be a villain but simply a character who has differing opinions than that of another. A character who is disagreeable is considered foil because two characters are contrasting in opinions or beliefs. A character who simply has contrasting taste in appearance or opposite personality could also be considered a foil character.
References
Frye, Northrop, Sheridan Baker, George Perkins, Barbara M. Perkins. The Harper Handbook to Literature. 2nd Ed. Addison- Wesley Educational Publishers Inc. 1997.
Harmon, William, Hugh Holman. A Handbook to Literature. 9th ed. Pearson Education, Inc. Upper Saddle River, New Jersey. 2003.