A dramatic foil is a secondary character whose own character traits are completely opposite of another character's. The dramatic foil is used in juxtaposition with another character, helping the reader to see their traits more vividly.
In Rome and Juliet, Mercutio serves as a dramatic foil for Romeo. We see this clearly in Act 1, Scene 4, in which Mercutio tries to persuade Romeo to join them in crashing the Capulets' feast. Romeo takes love very seriously, pining and yearning for Rosaline, while Mercutio treats love as a joke. When Romeo says he cannot join them because he feels too heavy-hearted, Mercutio tells Romeo to "borrow Cupid's wings, / And soar with them above a common bound" (I.iv.18-19) Furthermore, when Romeo argues that love is "rough," "rude," "boist'rous," and that it "pricks like a thorn," Mercutio advises,
If love be rough with you, be rough with love.
Prick love for pricking, and you beat love down. (I.iv.28-29)
All of Mercutio's light humor shows us that Mercutio does not treat love as seriously as Romeo, proving that Mercutio serves as a dramatic foil for Romeo.
Benvolio also serves as a dramatic foil for Tybalt. We see clearly in the first scene that Benvolio is a practical, peace-loving gentleman, while Tybalt is an arrogant, impetuous, and fiery tempered fiend. We see this when Bemvolio first appears on the set trying to break up the opening fight between the two houses' servants, saying "Part fools! / Put up your swords. You know not what you do," and trying to "beat down" their own swords with his own (I.i.59-60). When Tybalt arrives at the scene and sees Benvolio's drawn sword, he assumes that Benvolio is being heartless and callous enough to start a fight with the servant men. Tybalt even responds by saying, "What, drawn, and talk of peace? I hate the word," proving that Tybalt's violent anger serves as a dramatic foil for Benvilio's peaceful mind.
A dramatic foil is a character who contrasts with another character in order to make the traits of the second character evident. Mercutio is one dramatic foil to Romeo, because he objectifies women and tends to make lewd jokes, which contrasts with Romeo's romantic ideas. Benvolio is another character foil to Romeo, as he remains level-headed and breaks up fights, whereas Romeo tends to be more emotional, such as during his upset over Rosaline.