The content below is entirely editable.
"My life is elsewhere now." Volume 2, Chapter 9
Kazuna Takashiro is Chizuna's younger brother. Recently, he has become afflicted by a "vampiric" condition that is passed down through the Takashiro family. He has his mother's last name. According to Chizuna, he looks like their father.
After his mother's death, he was raised by family friends Shin and Natsuko Eda, who constantly ask to adopt him. Shin lies to him about his father's death and won't talk much about the Takashiro family.
Kazuna finds his old family house one day and meets his sister for the first time in many years. Chizuna tells him all about the family, including the fact that their father commited suicide six months prior. She warns him to leave and never come back or his life will be destroyed. Still, he is driven to know the truth and eventually goes to live with her and forsake his home with the Edas. For his disease Chizuna gives him two bottles, one of a serum that gives her some relief, and one for suicide, if it becomes too much.
At the first of the manga series, he admits to himself he's in love with Yaegashi, but later pushes her away after he has a dream in which he killed her for her blood. As Kazuna slowly cuts ties with Yaegashi, he begins to fall in love with his sister. The two even kiss (once in the anime, twice in the manga). For a long time, he skips school and abandons his friends in order to keep them safe. At the end, when Chizuna knows she is dying, she goes back to school and forces Kazuna to come with her so that he can try to integrate back into life. Since he is male, Chizuna believes that the Takashiro disease can be repressed within him and he can have a life again. In a climactic scene, Yaegashi forces Kazuna to drink her blood when he has another attack. Afterward, he runs from her.
At the end, with Chizuna near death, he declares that he will take the suicide medicine, because he can't think of living on after she dies. Later both are shown unconscious on the floor of Chizuna's bedroom. In the anime he dies. In the manga, in the next chapter he is shown in the hospital, with no memory of what has happened in the past year.