An ambitious, sly, snarky person who wishes to become Montague's heir. He is contemptuous of the common people and is charged by Montague to spy on Romeo. He also seems to have some degree of interest in Hermione, but whether he has honest feelings for her or desires to use her as a pawn in his possible plans remains debatable. Mercutio's desire for power appears to stem from his resentment towards his father, Titus, who doesn't live up to his son's expectations as a Montague advisor. Montague is aware of his disdain for his father and encourages - none too subtlety - Mercutio to kill Titus, but he doesn't do it.

A few days after Romeo's banishment, Mercutio agrees to be adopted as Montague's new heir but is unprepared for the bloody initiation, whose climax was Titus' death at the hands of Montague. Paralyzed by fear, Mercutio could not voice out for help nor assist his father's peril, and later swore his loyalty to the bloodied Montague. He tries to fit the mold of Montague's heir but, in truth, the event of his father's death has traumatized him and his guilt over doing nothing envelopes him. The obvious collapse of Montague's reign drives him to do what he thinks will get him the most praise: he stabs Montague and "saves" Juliet. Expecting to be called a hero, he is stricken when Tybalt dismisses him and leaves from the scene in mad laughter.

(Souce: Wikipedia)