Romeo is the ultimate "head in the clouds" lover. He moons over Rosaline, then he completely loses it over Juliet. He is the incurable romantic.
Benvolio is the voice of reason. He's the one who tries to remind Romeo that there are other fish in the sea when Romeo is depressed over Rosaline. He's also the one who tries to calm down violent situations, like the fights between the Capulets and the Montagues.
Mercutio is the hot-blooded cynic toward romance. He uses bawdier (sexual) humor in discussing females, almost as if that is his only use for women. He is markedly different from Romeo, who has a romantic ideal in his head concerning love.
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A foil character is one with opposite traits from a main character. A very careful and cautious character, for example, might be a foil to an impulsive, happy-go-lucky lead, warning him against jumping into trouble.
In Romeo and Juliet, Romeo is in love with love. At the opening of the play, he moons around heartsick over his unrequited love for Rosaline, and later he very quickly falls head over heels into love with Juliet.
The quick-witted, mercurial Mercutio acts as foil to Romeo by being as cynical about romantic love as Romeo is enamored of it. Mercutio scoffs at true love, advising Romeo not to let it control him. He tells Romeo in Act I, scene 4 that if love is rough with you, you should be rough to love and "beat it down." In Act II, he mocks Rosaline's "bright eyes" and "scarlet lips," and in the story of Queen Mab as they walk to the party at the Capulets', he frames love as lust.
Likewise, Benvolio is as level-headed and concerned for others as Romeo is the drooping and self-absorbed lovebird. He basically tells Romeo that there are a lot of beautiful women in Verona and urges Romeo to look around and find someone else since Rosaline won't respond. Unfortunately, the replacement Romeo chooses embroils him in a whole host of more tragic problems than mere unrequited love.
Romeo is self-absorbed and romantic. Mercutio seems to have negative feelings toward love. Benvolio is always looking out for others. He thinks before he acts and he is also the peace maker in the play.