Yeah, we'll be real: Lauren's brother Keith is a brat.
As Lauren says, Keith doesn't care about much (2.11). He's kind of a foil to Lauren: she's passionate and idealistic, but he just likes to "dodge work and dodge school and dodge church" (2.11) and pretty much any other responsibility that comes his way. He's twelve, and he's her stepmother's favorite (2.11)—but Lauren says he's the dumbest of the Olamina kids. His goal, if he has one, is to move to Los Angeles (2.11), not for any particular reason, as far as Lauren can make out, but just because.
Keith seems downright mean compared to Lauren—or compared to anyone, really. For instance, Keith "used to pretend to be hurt just to trick [Lauren] into sharing his supposed pain [due to her hyperempathy syndrome]. Once he used red ink as fake blood to make [her] bleed" (2.23). Yep. Mean guy. And it gets worse.
Keith wants to think of himself as an adult. So, he really wants to go to target-practice, for example, even though he's too young according to the age rule (4.42). Eventually, his whining pays off, and his parents get him a weapon (9.1). It isn't too much longer before he leaves Robledo for good and joins up with whatever wrongdoers he can on the outside to stay alive.
Keith's survival in the outside world is the topic of conversation between him and Lauren during one of the brief times he returns to Robledo. Lauren wants to find out how he's surviving, and he admits that he killed a man. Yep, we told you it gets worse. What's more is that Keith says shooting this guy to death didn't bother him (10.93). In a way, Keith is pretty antisocial (in the real meaning of the word, which means someone who doesn't abide by any of society's rules), which, when you think about it, makes him the opposite of Lauren, who's all about hyperempathy and building community.
All this antisocial behavior apparently ends up getting Keith killed. On August 26, 2026, his parents have to go downtown to identify his dead body (10.121). Turns out he had been gruesomely tortured prior to death (10.123)—a drug deal gone wrong, perhaps. Reverend Olamina uses Keith's death as a way to warn his other kids about how dangerous the world is beyond Robledo (10.124). Yet Lauren has to go outside eventually, too.
It seems that Keith is driven, to some extent, by an urge to differentiate himself from his father. Lauren calls Keith out on this, telling him: "Every time you look at him, you see yourself" (10.32). Keith doesn't believe that, but it seems true to us. By disobeying his father, Keith tries to prove that his father doesn't matter—hmm, a little contradictory there, eh? Because if he didn't matter, then why disobey him so spectacularly?