The Fourth Primogenitor is the world's strongest vampire that should only exist in legends, but he has emerged in Japan with twelve familiars to spread chaos. For the observation and obliteration of this fourth primogenitor, the government and Lion King Organization decided to dispatch an attack mage known as a Sword Shaman to observe his every move. For some reason, the one chosen for the observation was an apprentice Sword Shaman, Yukina Himeragi, who is just a middle-school girl! Wielding the strongest anti-demon spirit spear, Yukina arrived at Demon District, Itogami City, to find the fourth primogenitor. Surprisingly, the primogenitor is a high-school student, Kojou Akatsuki, who can't even remember how he became a vampire! Together Kojou unlocks his powers and protects his city from utter chaos!
Asagi Aiba is one of the main supporting character in this series. She is Kojou's friend and classmate. She is an expert programmer and works for the Manmade Island Management Corporation.