In Romeo and Juliet, how would you characterize Mercutio? Is he a believable character?
Mercutio is extremely intelligent, quite witty, very lewd and sexually suggestive, and also rather self-important and proud. He enjoys wordplay, using puns and all manner of literary devices in order to showcase his wit. For example, when he's trying to convince Romeo to come with him to the Capulets' party, Romeo tells him that love pricks like thorns do. To this, Mercutio replies, "If love be rough with you, be rough with love. / Prick love for pricking, and you beat love down" (1.4.27-28). He means that if love has pricked Romeo and injured him, then he should prick love back; however, prick is a slang term for a penis. Therefore, he really tells Romeo to have sex in order to satisfy and/or deflate his desire.
Further, when Tybalt comes to challenge Romeo, Romeo will not fight him because he just married Juliet in secret that morning. However, Mercutio views Romeo's refusal, in the face of Tybalt's insults, as a "dishonorable, vile submission" and fights Tybalt instead (3.1.74). He is fatally wounded when Romeo comes between them, and he dies cursing both houses of Montague and Capulet.
Is he believable? Sure, as much as any of the other characters are. In the manner of many teenage boys, he's very interested in sex, rather full of himself, and impulsively reckless.