Strike the Blood, Vol. 1 (light novel): The Right Arm of the Saint

He never asked for this power. Kojou Akatsuki used to be an ordinary high school boy—but that wasbefore 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 whenever he gets aroused...