Mex

 is a slaver guarding the gate to The Pitt around 2277.

Daily schedule
Mex and three of his men keep their guns trained on the road into town all day, ready to blow away any pests or vermin trying to get in.

Interactions with the player character
He will take all the items in your inventory (except the "hidden weapon" you may have received from Werhner and the Slave Outfit) and then tell you to enter Downtown.

Behind the Scenes

 * Mex is most likely a reference to the chain of Mad Mex restaurants, which are relatively popular in Pittsburgh, and the surrounding area like in Monroeville and Squirrel Hill.

Appearances
Mex appears in The Pitt add-on for Fallout 3.