This is the scary guy who tries to kidnap or kill The Boy. He grabs The Boy and holds a knife to his throat, leading the The Man to suddenly becomes a gunslinger and shoot him square in the forehead.
The Roadrat only makes a brief appearance in the novel, but he's still important. Watching The Man go up against the Roadrat, we start to wonder if our hero is actually a good guy. We start to question his ethics. Is The Man really any different from these men who steal and kill? Is The Boy really a warrant for everything The Man does?
This [The Roadrat] was the first human being other than the boy that he'd spoken to in more than a year. My brother at last. The reptilian calculations in those cold and shifting eyes. The gray and rotting teeth. Claggy with human flesh. Who has made of the world a lie every word. (118.1)
The Roadrat is cast as a possible double for The Man. Certainly The Man seems to be a much better human being, but it's worth noting that we begin to doubt his character through the showdown with The Roadrat.