Mercutio has two principal roles in Romeo and Juliet. First, he acts as a foil to Romeo. While Romeo is the romantic lover who takes his passion seriously, Mercutio provides comic relief by using it as the subject matter for his satirical wit. Until act 2, scene 4, the two are unequal sparring partners due to Romeo's melancholy, but their dialogue at this point in the play shows that Romeo is just as clever as Mercutio, who is delighted to see him restored to his regular form.
Mercutio's other role lies in his centrality to the plot. Although he seems to spend all his time with Romeo and the Montagues, Mercutio, as "kinsman" to the Prince, is one of the few aristocrats in Verona, aside from Paris and the Prince himself, who is above the quarrel between the Montagues and the Capulets. His murder is thus an exceptionally serious event, which puts Tybalt seriously in the wrong and functions as a pivotal moment in the play. His closeness to Romeo and his bitter reproaches as he dies make it feasible for Romeo to be seized with rage and kill Tybalt. This combination of personal and social qualities in Mercutio is essential in ensuring that Romeo is punished by banishment—but not by death—for the killing of Tybalt.