All You Need Is Kill - Hiroshi Sakurazaka

When the alien Gitai invade, Keiji Kiriya is just one of many raw recruits shoved into a suit of battle armour and sent out to kill. Keiji dies on the battlefield, only to find himself reborn each morning to fight and die again and again. On the 158th iteration though, he sees something different, something out of place: the female soldier known as the Bitch of War. Is the Bitch the key to Keiji's escape, or to his final death?