Mercutio is Romeo's best friend he is neither Montague of Capulet. He attends parties and hang's out with Romeo and his cousin Benvolio. Romeo get's involved with Juliet who is is capulet and he is montague this causes a war to go on between the houses. Tybalt attack's Mercutio and Romeo eventually Mercutio dies in Romeo's arm's. this angered him so he avenged his best friend's death by killing Tybalt.
as he appears in Romeo + Juliet