Zahra plays a big role in the novel, but at first, Lauren doesn't really know her that well. She's just one of Richard Moss' three wives—and according to Lauren, Zahra is the "youngest and prettiest" (4.29) of those three. She has only one kid, Bibi, who is shot to death during the destruction of Robledo (14.90 and 15.6). After that tragic incident, she joins up with Lauren and follows her all the way to Bankole's land, where the crew founds Acorn.
Zahra has a scary background with Richard. Basically, he purchased her with money and totally controlled her like a slave (16.53-54). She explains this background to Harry:
Zahra told how Richard Moss had bought her from her homeless mother when she was only fifteen [...] and brought her to live in the first house she had ever known. He gave her enough to eat and didnʼt beat her, and even when her co-wives were hateful to her, it was a thousand times better than living outside with her mother and starving. Now she was outside again. In six years, she had gone from nothing to nothing. (15.12)
Zahra turns out to be a huge help to Lauren on the road north. That's because Zahra has had the experience of living on the streets, so she's able to give Lauren survival advice. For instance, Zahra recommends sucking on a little rock while walking to defeat thirst pangs (16.4). It's unfortunate that Zahra and Lauren didn't team up prior to the destruction of Robledo—maybe they could have kept the hometown afloat if they'd met sooner. But it seems Richard Moss was in the way of that, preventing Zahra from socializing with others.
The ol' Z definitely digs Earthseed: Lauren calls her the second convert (18.83). Zahra says, "[I]f you want to put together some kind of community where people look out for each other and don't have to take being pushed around, I'm with you" (18.83). That's how Zahra sees Earthseed. She doesn't really care about the Destiny—"I don't care about no outer space" (18.83)—but other than that, Zahra is game.
On the way north, Zahra romantically pairs up with Harry (17.32). Seems one of the main reasons for that is that he rescued her from rape during the destruction of Robledo (14.104).
All in all, Zahra was pretty ignored back in Robledo, but once she's out of that town, she turns out to be one of the most helpful people to have around.