Strike the blood. 1, The right arm of the saint

"Kojou Akatsuki used to be an ordinary high school boy--but that was before he became the Fourth Primogenitor, the most powerful vampire of all time. Worse--he doesn't even remember how it happened. Middle schooler Yukina Himeragi, meanwhile, is the Attack Mage sent to observe him--and to stop him if he gets out of control. But was it really a good idea to send a cute girl to observe a massively powerful vampire boy whose abilities activate whene...