In literature or fiction in general, a foil is character who draws attention to the distinctive qualities of another character (usually the protagonist) through contrast. Often, the foil will be completely opposite the protagonist or exhibit a protagonist's quality to a lesser extent. Sidekicks are the most common embodiment of foils, although rivals, romantic interests, and minor static characters also perform this role.

There are many famous examples of foils:

  • Batman and Robin - The Boy Wonder is prime example of a sidekick foil. Robin usually ends up needing help (like getting lowered into a vat of acid), which gives Batman a prompt to look heroic (like swinging across a pit of spikes and laser beams and saving Robin at the last second). Also, Robin's usual ignorance of the bad guy plans highlights Batman's insight of seeing right through them.
  • Sherlock Holmes and Dr. Watson - Poor Dr. Watson is another example of a foil. In his confused stupidity, he gives Mr. Holmes every opportunity to show off his intelligence more so than he usually would. Watson asks a stupid question and Holmes replies with "Elementary, my dear Watson," then goes on to explain the plot. Watson's foil role makes Holmes look that much smarter .
  • The Fool and King Lear - In Shakespeare's play, King Lear, madness plays an important role. The Fool, a jester type character, acts like he's insane by default by speaking in riddles and barely understandable phrases. However, his craziness doesn't compare to King Lear's disturbed ravings later in the play. This is an example of a foil highlighting a negative characteristic in a protagonist.
  • Darth Vader and The Emperor - This famous evil duo from Star Wars shows a different type of foil. It's obvious both Vader and the Emperor are evil, but to emphasize just how evil the Emperor is, Vader takes advantage of an opportunity to redeem himself near the end of the Return of the Jedi. The Emperor shows that Vader wasn't all bad; he had potential for righteousness.
  • The killer and the virgin girl that survives - This is more a general theme of foils. In modern slasher movies (Halloween and Friday the 13th), the fact that the killer slices up a bunch of teens having sex while the virgin girl survives draws attention to her purity. By remaining chaste, she is rewarded by not being maimed by the sequel spawning psycho.

Source: Murfin, Ross. The Bedford Glossary of Critical and Literary Terms. Boston, MA: Beford, 1998.