Date A Live, Vol. 8 (light novel) by Koushi Tachibana
Can you guess which one is me?'AEu The Seventh spirit, Natsumi, can seemingly transform at will and masquerade as anyone, even people Shido know very well. His only option is to date everyone until he can figure out who Natsumi is pretending to be'AEbut there'AEos twelve whole suspects!