I have included some links to literary criticism centering on the character of Mercutio. Hope they help! Be sure to cite your sources should you use the information you find in these cites.
You shouldn't have any trouble writing about mercutio, as has been written here before, Mercutio is a mercurial (changing) character, who goes from joking to serious, and who is a great friend to Romeo. From the minute we first meet Mercutio he is a comic foil to Romeo. In Act one Scene 1 when Romeo is upset about Rosaline, Mercutio makes him laugh and cheers not only him up, but the audience as well. When he is onstage he tends to be the center of attention, mostly because of his wit. Mercutio's changeable nature shows in the fatal marketplace scene. At one moment he is joking with Benvolio about quarreling, and the next moment he is quarreling in deadly earnest himself. He had hoped to see Romeo answer Tybalt's challenge to a duel and is disappointed by what he sees as Romeo's cowardice or submission. Mercutio has to fight, because Romeo will not. The most important speech Mercutio makes is after he is fatally injured in the fight with Tybalt. He says, "A Curse on both your houses" Mercutio then dies. His serious ending is completely different than how he entered the play, as a comic foil.