Strike the Blood Novel Vol. 1 The Right Arm of the Saint (novel)

Kojo Akatsuki used to be an ordinary high school boy—but that was before he became the Fourth Progenitor, 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? And on an artificial island populated with all manner of demonic individuals, each with their own designs on the Fourth Primogenitor—!