In fiction, a foil is a character who contrasts with another character (usually the protagonist) in order to highlight particular qualities in their counterpart (the term derives from the practice of backing gems with foil to increase their brilliance). In some cases, a subplot can be used as a foil to the main plot, particularly in metafiction (‘breaking the fourth wall’) and the ‘story within a story’ motif. A foil often differs drastically from the lead character, but can also be extremely similar, with only a key difference setting them apart. Foils generally serve one or more of three broad functions: contrast (‘this is different than X’), exclusion (‘this is not X’), or blame (‘X did this’).
In ‘Frankenstein,’ by Mary Shelley, the two main characters of Dr. Frankenstein and his ‘Adam of your Labors,’ his ‘creature,’ his ‘wretch,’ are both together literary foils. Both are hungry for knowledge, but whereas the doctor is selfish and arrogant, the monster is compassionate and gentle. 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 ‘Julius Caesar,’ the naive Brutus has foils in Cassius and Mark Antony, who are ambitious and experienced politicians.
The Daily Omnivore 