Pronto

 is a slaver who works in Paradise Falls, repairing and bartering weapons and ammo.

Unmarked quest: Economics of Violence
You can expand Pronto's shop by giving him 20 Chinese Assault Rifles. In addition, he will get a bit more cash (around 400 instead of 150) and you will be privy to a 20% discount.

Appearances
 only appears in Fallout 3.