Mercutio's very name says a great deal about what he is like as a character. Mercutio is derived from the word mercury, which refers to a poisonous metal that changes very quickly under temperatures much lower than needed to change the properties of other metals. Since mercury is a metal that changes so easily, we also derive from it the adjective mercurial, which means changeable. In addition, mercury was also the name of the Roman god Mercury, a messenger god "of eloquence" (eNotes, "Romeo and Juliet: Mercutio (Character Analysis)"). The name mercury greatly reflects Mercutio's characterization in that he is hot tempered, impetuous, and easily provoked. But he also reflects the god Mercury through having an eloquent and profuse wit.
While Mercutio's wit is seen in his banter and jokes all throughout the play, especially his sexual innuendos, we see his impetuous, quickly changing, fiery temper in Act 3, Scene 1. In the very beginning of this scene, Benvolio, our rational character, begs Mercutio to get off the street, knowing that if they ran into any Capulets, they would not be able to escape a fight. Mercutio proves his impetuousness, his propensity to make rash emotionally driven decisions, by refusing to listen to Benvolio's advice. Then, when Tybalt does show up and challenges Romeo to a duel, Mercutio shows his hot, easily changing, fiery temper when he becomes disgusted by Romeo's attempt to pacify Tybalt and challenges Tybalt to a duel himself, as see in Mercutio's line, "O calm, dishonourable, vile submission! ... Draws.(it) Tybalt, you ratcatcher, will you walk?" (III.i.72-74).
While we know a great deal about Mercutio's sense of humor and personality, we are never really told much about his looks. However, if we can believe what Mercutio says about himself, he might be rather plain. Mercutio refers to his own plainness in the scene in which he and Benvolio are trying to persuade Romeo to crash the ball:
Give me a case to put my visage in.
A visor for a visor! What care I
What curious eye doth quote deformities? (I.iv.30-32)
The first line in this passage can be translated as Mercutio asking for a mask. The rest of the lines can be understood as him saying what does he care about masks; what does he care about people making note of his deformities, or blemishes (eNotes). However, we can also wonder if he is really being serious here. His sexual innuendos characterize him as a typical play boy, and charismatic play boys like himself are generally good looking.
Mercutio's character is that of a carefree young man that likes to joke around, laugh, and tease others. He likes to be the center of attention in many scenes. He also likes to get the last laugh with others. He can also be impulsive and instigative when it comes to the fighting scenes and teasing of the Nurse.