Latchkey

Isabel, once known as Wasp, has become leader of the fearsome upstarts, the teen girl acolytes who are adjusting to a new way of life after the overthrow of the sadistic Catchkeep-priest. They live in an uneasy alliance with the town of Sweetwater—an alliance that will be tested to its limits by the dual threats of ruthless raiders from the Waste and a deadly force from the Before-time that awaits in long-hidden tunnels.
Years ago Isabel befriended a nameless ghost, a supersoldier from the Before-time with incredible powers even after death, and their adventure together in the underworld gave her the strength and knowledge to change the brutal existence of the Catchkeep acolytes for the better. To save Sweetwater, Isabel will have to unlock the secrets of the twisted experimental program from centuries gone by that created the supersoldier and killed his friends: the Latchkey Project.