A foil is a character that is used as contrast to another character. Foils are completely different from one another. They are used to highlight and intensify the different qualities of each other.
Romeo and Mercutio are extremely different characters. Romeo is sensitive , serious, often focusing on the negative, and tends to fall in love quickly and easily. He is a hopeless romantic. Mercutio takes nothing seriously. He makes light of everything and constantly makes a joke out of everythng. He is upbeat and funny, light-hearted. You'd never catch Mercutio falling for someone. He'd never let it happen.
Benvolio is the peace-maker of the play. He always tries to break up any conflicts that occur. He is calm and benevolent (kind). It take a lot to upset or anger him. Tybalt, on the other hand, is violent and agressive. He has a bad temper and will fight at the drop of a hat. It does not take much to incite him to violent behavior.