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.

Effects of player actions

 * If you tell him you're a returning escaped slave, he will take all the items in your inventory (except the "hidden weapon" you may have received from Werhner), give you a Slave Outfit which is automatically equipped (whether you're wearing one already or not), and then tell you to enter Downtown.
 * If you tell him you came looking for work, he'll invite you to enter through the door to Downtown and fill out an application, whereupon you are ambushed and all your items are taken (except the "hidden weapon" you may have received from Werhner), and you are issued a Slave Outfit which is automatically equipped.
 * If you kill him on the way in, you will also be ambushed, with the same result as if you were trying to "apply for a job" (but with different dialogue from the ambushers).

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.