A foil is a character who contrasts with another character—usually the protagonist—in order to highlight particular qualities of the main figure. The differences throw the central character into sharper relief.

In Hamlet, Laertes and Fortinbras serve as foils to Hamlet: both are young men avenging fathers, yet their decisive action contrasts with Hamlet’s delay. Watson is a foil to Holmes; his ordinary perceptions make Holmes’s brilliance more visible. The foil need not be an opposite in every respect; even a few salient differences can do the work. Sometimes the foil is a friend, sometimes a rival, sometimes a minor figure who appears only briefly.

Writers use foils to avoid having to state a character’s traits directly. The contrast shows rather than tells. A well-drawn foil has enough independent life to remain interesting; a purely mechanical foil feels like a device rather than a person.

When one character exists largely to set off the qualities of another, that character is a foil.