From Yen Press:
Crafted from the corpses of battlefield casualties, a race of immortal soldiers called "The Undying" fought a bloody war for their human creators. When the war ended, they were eradicated in the name of God at the hands of those for whom they had fought endlessly...
Eighty years later, an orphaned girl named Kieli questions if that God even exists. Unable to bear the hypocrisy of the living, spirit-sensitive Kieli spends more time with ghosts ignorant of their own deaths rather than with girls her own age. But when she runs into a young man named Harvey who can also see ghosts, Kieli thinks she's finally found a friend among the living. She resolves to be part of his life, but the truth of Harvey's existence will throw Kieli's world into chaos and force her to reevaluate the meaning of faith.