He's the only human in the Kirishiki househould. His job is taking care of the properties. Seishirou originally came from a distinguished family, but he found his parents vile and disgusting. Chizuru killed them both, but refused when Seishirou asked her to turn him into a vampire too, pointing out that since his parents didn't rise, he probably couldn't either. Since then, Seishirou has been with the Kirishiki family and thinks of them as his own, and occasionally gives blood to Chizuru.
When Chizuru is killed by the village doctor Ozaki Toshio, Seishirou attempts to shoot Ozaki in revenge, but is attacked by Natsuno. Natsuno then orders Seishirou to attack his vampire family. In the anime, he manages to shoot Yoshie to death and eventually perishes in the fire that engulfs the entire Sotoba village. In the manga, he also shoots Yoshie in the head (which doesn't kill her), and kills himself before he can harm anyone else of his vampire family.