Mirror's Edge (Impostors, Book 3) - Scott Westerfeld - Bog

Raised to be her twin sister Rafia's body double and bodyguard, but now with new face and body, Frey and her lover Col are determined to return to Shreve to rescue Boss X, a friend and ally in the rebellion of the free cities against people like Frey's father, ruthless ruler of Shreve. But Frey is troubled by things even beyond the dangers of her mission: confusion over her own identity, guilt over having killed her brother (even if he was trying to kill Rafia at the time), and worry about whether her sister, who has stolen Frey's name and identity, can be trusted with the fate of Shreve.