In Romeo and Juliet, Benvolio and Tybalt are foils.
Benvolio is a peace-maker, one who tries to part the two families as they brawl in the streets of Verona. Tybalt, on the other hand, is an instigator and a hot-head; he starts much of the fighting. The two have a brief exchange in Act I:
I do but keep the peace: put up thy sword,
Or manage it to part these men with me.
What, drawn, and talk of peace! I hate the word,
As I hate hell, all Montagues, and thee:
Have at thee, coward!
Both are also foils of Romeo. Romeo will play both roles in Act III. Now that he is married to a Capulet, Romeo will play the part of Benvolio as peace-maker and try to part Mercutio and Tybalt. But, after Tybalt kills Mercutio, Romeo will become a hot-head like Tybalt, demanding vengeance.