The most well-known dramatic foil character in "Romeo and Juliet" is Mercutio. He acts as a direct opposite of the protagonist, Romeo. Mercutio is a skeptic, while Romeo is a romantic. Foils are used to bring emphasis to the qualities of another character, and they often serve as comic relief.
Mercutio mocks Romeo for his love of a Capulet, even though Romeo is his friend. He has a sharp wit and a sense of humor, while Romeo is serious and easily heartbroken. Mercutio has no allegiance to either the Capulets or Montagues, and he tries to keep the peace between both families.