Hunter. The Valiant Series, #1

A prisoner by her own choice, Averella enters the prison known as Zagerah. There, the oppressive government throw innocent men and boys to either be killed by the dangerous environment or be brutally murdered by the Hunters-magically enhanced assassins with only one objective: to kill the prisoners. The government never takes women, so Averella disguised herself as a man to save her sickly brother. As a Hunter, Jared has only ever known the life of a killer.
Under orders, Jared disguises himself as a prisoner to get closer to the newest inmate and unravel the newcomer's secrets. With her secret on the verge of discovery and no sign of her brother, Averella must evade the Hunters while she scrambles for survival. But how can she save her brother when she can't even save herself?