Who are some character foils in Shakespeare's Much Ado About Nothing, and what is their effect on the story?
Foils are characters that contrast with one another so that the author can highlight important details or traits of each person. In Much Ado About Nothing, Shakespeare implements many different types of foils. He even uses groups of characters as foils for one another. I will give you a few examples--but there are many more.
First and most importantly, our two main couples, Beatrice and Benedick and Hero and Claudio, are foils for each other. We can see this starting in the first act. Beatrice and Benedick engage in witty and argumentative repartee and seem to completely hate each other. In contrast, at the beginning of the story Hero and Claudio are obviously in love, and they act kindly toward one another. In Act IV, when the wedding is broken up by Don John's lie, the two couples contrast yet again as Benedick and Beatrice unite and Claudio and Hero are torn apart by the horrible accusation that Hero has been unfaithful.
Another important pair of foils in the story is Don Pedro and Don John. These two men exemplify a more classic version of the foil. One is good and one is bad. From the beginning of the story it is made clear that Don John is bad news. In Act 1 Scene 3 he is seen conspiring against his brother with his henchmen because he would much rather be known as "a canker in a hedge than a rose in his [brother's] grace." It is also made clear in this scene that Don John was already granted mercy by his brother Don Pedro for previous wrongdoing. It is clear that one of these men is honorable and good and the other is not.
There are numerous other foils in this story and I encourage you to look for them! Think about characters whose ideas, personalities, and actions contrast drastically.