Finding a single foil has always sort of bugged me when a teacher asks for that kind of thing. The reason is that a foil is a character that shows qualities that are in contrast with the qualities of another character. The point is to highlight the traits of the other character. The reason finding a single foil doesn't make sense to me is because by its very definition you need a pair of characters. If character "B" is a foil to character "A," then logically "A" is a foil to "B."
So for "Much Ado About Nothing," I'll go over some foil pairs. I think that Beatrice and Hero are foils to each other. Beatrice is intelligent, lively, and incredibly witty. She also is jaded when it comes to love, because she has been divorced. Hero, on the other hand, is much more quiet and modest. She is a bit idealized in the play as that standard perfect beauty.
Claudio and Benedick are also foils to each other. Claudio is a young, impetuous soldier while Benedick is more of a seasoned aristocrat of a soldier. Benedick is adamant that he will never fall in love, while Claudio immediately falls in love with Hero. Benedick is incredibly witty and intelligent, while Claudio simply doesn't display that kind of wit. Perhaps because of his youth, Claudio is also easily fooled and manipulated as illustrated by Claudio believing anything that Don John says about Hero. Contrast that with Benedick who stays loyal to Beatrice.