The Bride of Demise, Vol. 1

A VOW THAT CANNOT BE BROKEN!
It's been years since the kihei appeared without warning and overran the earth. To combat this threat, humanity established the Twilight Academy to train students as soldiers, though most die before graduation. During a mission, Kou Kaguro's squad is attacked, and he must face his fate as yet another student casualty...or so he thought. A girl in white appears during his final moments and swears to protect him, filling a hole in his heart and setting them both on a path of love and despair.