In Romeo and Juliet who is Mercutio?

Mercutio is related to the Prince, and he is Romeo's friend. Mercutio has a huge ego, jokes around a lot, and is a good sWordsman. In the play, (in his main scene) Mercutio fights Tybalt, Juliet's cousin, even though Romeo and Benvolio (another Montague) disagree, Tybalt, with equally good fighting skills and same huge ego, fights Mercutio after the Prince told them "If you ever disturb the quiet of our streets again, your lives will pay for the fore fit of the peace." (meaning that they will be killed for combat) Finally, Romeo tries to break up the fight, but instead gives Tybalt a chance to stab Mercutio, for Romeo blocked Merutio's view. As he is dying he yells "A plague o both your houses, they have made worms meat of me, and soundly too, your houses!" He blames them even though he started the fight in the first place. Mercutio is also shown to be mad, as shown in his Queen Mab speech.