Natividad is the mother of the mixed family whom Lauren helps at the water station. Her child is Dominic, a.k.a. Domingo, and her man is Travis. When she first teams up with Lauren, Natividad tells her they're on their way to Seattle for work that pays money (17.153). But they end up joining Lauren's group and staying at Acorn with them on Bankole's land.
So who's Natividad? What's she all about? Well, Lauren says she's "a pale brown woman with a round, pretty face, long black hair bound up in a coil atop her head" (17.152). But that's just physical description. What's more interesting is that she asks to hear some of Lauren's poetry (17.173) and says that she's really concerned for her child's wellbeing (17.151). She asks interesting questions about Earthseed—like whether God is male or female (18.57). All in all, she rocks.
What's her background? She was the maid of a rich family, and she met Travis, the son of the cook, while working there. They got married but took off once the master started scoping out Natividad as a possible victim to rape. (18.42-46) Not your typical sci-fi heroine with blasters and a divine or royal family lineage, right?