Joe Boot is Elmira's son from her previous marriage. He's also July's "second deputy" (26.43). Elmira basically disowns him, leaving him to travel with July.
Joe is a good foil for Newt. Both are disowned by their parents, both are nervous at the beginning of their trips, and both have opportunities for growth. Joe is offered a job from both a creepy-crawly bug man named Sedgwick ("What was there to study about a bug? Either it bit you or it didn't" [38.30]) and Wilbarger. But he doesn't take either position.
This is what makes Joe different. He doesn't have the same success Newt does, and he's killed by Blue Duck, which means that he tragically never will. What makes Joe and Newt different? Is it simply luck? Or was Joe always fated to die? It's interesting to think about, because if Joe had taken a job with Wilbarger, he'd still have been killed.