Yukina Himeragi is the female protagonist of the series. She is an attack mage, or in the Lion King Organization, a Sword Shaman, who is sent by the Lion King Organization to observe the Fourth Primogenitor, Kojou Akatsuki. As she's appointed to observe him, she became a middle-school student studying in Saikai Academy who follows him everywhere and is also ordered to kill him if he becomes too powerful and insane to control himself.

Spoiler

She is also the first blood mate of Kojou, and thus being the first, her blood smell is always associated with Kojou's own smell, which has been pointed out many times by other characters who could smell blood, probably due to her blood interbreeding with Kojou's as she was the one who activated Kojou's first 'real' vampiric act. Also, unknown to her, she was intended and prepared by the Lion King Organization to be his lover.