Deputy Joe Wallace was a member of the Gunnison County Sheriff's Office. In 2004, he became involved in the Xenomorph outbreak that ultimately led to the destruction of the town.
Joe routinely investigated the town's sewers and attempted to move the homeless people who dwelled there, before Harry's dog brought in Buddy Benson's melted off arm. This then caused a massive search party to look for Buddy in the woods.
Joe's full name is never mentioned in the film or any related promotional material; it is only given in the shooting script for the film.