Zeb was another unlucky kid captured by the DHS and thrown in Gitmo-by-the-Bay. And, when his appendix burst in prison, he met Darryl in the infirmary. Zeb was questioned for three months and then became Darryl's cellmate.
When Zeb meets Marcus he's somehow escaped (we never find out the details). He looks homeless…and more than a little traumatized by his stint in Gitmo-by-the-Bay.
But Zeb gives Marcus hope and news of Darryl, a key moment in the plot. Zeb's character is a way for Doctorow to introduce issues like alternative economies (like being freegan) and ways of being that aren't just rebellion-driven (like Marcus). And Marcus needs to go underground, Zeb's proves once again that he's a useful guy to know. He's the definition of streetwise—he has connections and an amount of intuition that balances out Marcus' more aggressive and intellectual outlook on life.