Often considered a comic foil. (A foil is a character who by strong contract underscores or enhances the distinctive qualities of another character.) to his friend Romeo, Mercutio commands attention when he speaks, but comes across as likable during his witty and light-hearted banter with Romeo; however, there is another side to his character. Mercutio’s name is related to words such as mercurial, which means changeable, and mercury, a poisonous element; these words also represent another view of Mercutio’s personality: volatile and explosive.
When he is alive, he has a teasing quick wit and is swift to make bawdy, raucous and inappropriate comments, such as his teasing of Romeo to improve his mood when he is love struck by Rosaline and again in his conversation with the nurse who is looking for Romeo. It is his volatile nature, however, that proves to be deadly not only to himself but eventually to others in the fatal marketplace scene. In an instant he moves from joking with Benvolio to fatally quarreling. He finds himself so severely disappointed by what he views as Romeo’s cowardice in refusing to meet Tybalt’s challenge that he jumps in to meet it himself only to be mortally wounded when Tybalt takes unfair advantage of Romeo’s well-intentioned interference. On the verge of dying, Mercutio venomously wishes a plague on the houses of both the Montagues and the Capulets , revealing the extent of his bitterness through the repetition of this curse not once, but three

Mercutio is somewhat like his namesake, Mercury. He is mercurial, which is to say, he changes quickly, at least verbally, and he likes to spin ridiculous and extended stories. If he were more mean-spirited, you might think of him as liking to, um, "BS" people. Mostly, though, he likes to joke around.

When Romeo sighs over love, Mercutio tries to bring him down to earth. He also shifts the talk from lofty love to pretty earthy sex.

When he's alive, he is comic relief and Romeo's friend. In his death, he changes everything. Tybalt kills him, and Romeo kills Tybalt. This gets him exiled, and starts everything down the final road to ruin.