In literature, foils exist to emphasize the contrasts between two different characters. In Shakespeare's Much Ado About Nothing, Beatrice and Hero make interesting foils to one another thanks to their very different styles of self-expression.

Though Beatrice is meant to be a sidekick of sorts to Hero's lead role, Beatrice's strong personality and voice dominates. Beatrice is witty and fiesty, and her style of expressing her thoughts and emotions often involve humor and sharpness. Hero, on the other hand, is demure and gentle in her manner and her communications with other characters; she follows instructions without questioning anyone's authority over her. Furthermore, Hero speaks so poetically that it is impossible to imagine her taking Beatrice's sharp tone with anyone in any situation.

The nature of these foils may have an interesting impact on the audience; for example, audience members may wonder if Hero is a paragon of sweet femininity or if she is simply a beautiful, but bland, young woman, while they wonder about the stereotypes of womanhood defied by Beatrice, with her fiery wit and her hilarious sense of humor.

Well, here's the problem: I'm not sure they both are foils, primarily. Beatrice is one of the major comic engines of the play. She's so witty! Hero is a much simpler character. She's so idealized that she is almost flat. A foil plays opposite the main character, and interacts with him/her to show the main character's traits better.

I guess Beatrice could be a foil for Benedict; the two of them are foils to each other for their wit, the way they both stand apart from love, and the way their egos let them be manipulated.

Hero could be a foil to Beatrice more easily. Where Beatrice is lively, witty, and divorced from love, drawing attention through her wit, Hero is quiet and virtuous, and draws attention through her modesty. They are thus foils to one another.