I think that Mercutio is a foul to Romeo because he tries to change Romeo's way of thinking. In Act 1, Scene 4, line 53, Mercutio says that "dreamers often lie" when he is asked about his own dreams. Romeo seems to think positively about dreams/ dreaming, however, Mercutio disagrees and seems to think that they are lies. Mercutio's personality is very light-hearted and kind, whereas Romeo's is somber and serious. These contrast each other, which causes Mercutio to be a foil. After the party, Mercutio proceeds to tease Romeo about his love life, making it seem like a joke again. Romeo truly believes he is in love and is serious, and Mercutio takes it all as a joke.