Characters who have qualities that are in sharp contrast to one another, thus emphasizing the traits of each, i.e characters who are opposites of one another in a way that you are supposed to notice or pay attention to. To be character foils, two figures must have enough in common for you to compare them, but must be pointedly different in at least one way. Be careful with the use of this term–while foil can be a verb, in writing about literature it is almost always used as a noun.