I read this probably in the late 90s or early 2000s.
The protagonist is a teenage boy, a member of the slave race. He may or may not be slightly ostracized among his own people because he looks like a member of the master race?
Every night the slaves have to enter the city to perform maintainence while the inhabitants are asleep. I think the master race used chips embedded in their bodies to enhance their abilities. The boy finds a teenage girl in the city (named Ari?) who's been kept alive and about to be killed for some reason. Once leaving the city and cut off from their technology, her mental state reverts back to that of a baby and his parents raise her as their own daughter. Possibly a series of books with the second being partially in the POV of the girl?
No luck on Google... any help appreciated, thanks!