Travis and his family come into the story in a pretty spectacular way. The opportunists or "coyotes" who hang around water stations looking to take advantage of people go after him, Natividad, and their child—but Lauren trips the attacker and saves the day (17.51).
After that, the two groups—that's Lauren and Zahra and Harry on one side, and Travis, Natividad, and their kid Dominic (a.k.a. Domingo) on the other—are kind of wary of each other for a while, hiking north and camping in close proximity to one another; but eventually, after more help from Lauren (17.129-133), the groups team up, adding numbers to Lauren's wonderful migrating Earthseed crew.
So that's the story of how Travis gets involved, but who is he, really? Well, let's start with some basics. The dude has a "deep-black complexion" (17.152), he's "young, good looking, and intense" (17.152), and he's a "stocky, muscular man" (17.152). He's all that good stuff. But more importantly, he's the person Lauren calls her "first convert" (18.83). He argues with Lauren about her beliefs, but she admires his persistence in doing so (18.82), and she's eventually able to win him over.
Travis doesn't get all bent out of shape about whether Lauren's ideas are "realistic" or not. As Lauren puts it, "He didn't point out that a person walking north from L.A. to who-knows-where with all her possessions on her back was hardly in a position to point the way to Alpha Centauri" (18.82). He seems to recognize that ideals are important, and so he gives Lauren a good hearing.
Maybe Travis is so open to Lauren's ideas because he, like she, is into reading and writing and books and all that, kind of like we are here at Shmoop. Travis says his mother "taught [him] to read and write" and "taught [him] to teach [himself]. The man she worked for had a library—a whole big room full of books" that she would smuggle him (18.37-42).
Hey, the sky's the limit when you're a reader.