Like Bardolph and Nim, Pistol is another "cony-catching rascal" and a member of Falstaff's crew of degenerates. Just as his name implies, Pistol has an explosive personality and a really, really, really big mouth. The guy loves to talk smack to just about anyone willing to listen.
Allow us to demonstrate. The first thing out of his mouth in this play is "How now, Mephistopheles!" (1.1.109). Translation: "What's up, Punk! You want a piece of me?" That's what he says when Master Slender accuses him of stealing from him. (By the way, we're pretty sure Pistol picked Slender's pockets but we're not expecting a confession any time soon.)
Of course, Pistol speaks (okay screams) his most famous lines after Falstaff refuses to loan him some money:
Why, then the world's mine oyster.
Which I with sword will open. (2.2.4-5)
Okay, Pistol. You say you're ready to rumble and use force in order to get your piece of the pie but, the funny thing is, we never actually see you throw down with anyone in this play. Sure, you talk trash like you're gearing up for Jim Rome's Smackoff, but you back down pretty quickly when someone gets in your face. What's that? You've got nothing to say? That's what we thought.
In fact, the only significant thing Pistol ever does is to help his buddy Nim rat out Falstaff for trying to hook up with two married women. Shakespeare's point? Pistol is all TALK. Also, there really is no honor among thieves.
P.S. You can check out all of Pistol's hilarious lines here. We're estimating that over 90% of the guy's lines are outrageous insults and ridiculous, empty threats.