Saya's adopted "elder" brother. Once a popular student at school and a star athlete, he quits playing baseball shortly before Saya's arrival and has been getting into more fights at school. His father's death forces him to grow up and try his best keep his family united. He struggles greatly over Saya, and later Riku, not being human. As he finally comes to terms with both, Riku is raped and killed by Diva, and Saya disappears for a year. As the last surviving family member of Saya's adoptive family, he has sworn to protect her and return to Okinawa with her once everything has been settled. Towards the end of the series he convinces Haji to confess his feelings to Saya.

Initially Kai has little skill in fighting or marksmanship, but he improves as the series goes on. When George is kidnapped, Kai steals his father's M1911 pistol to try to rescue him. David confiscates it, but later returns it to show that he feels Kai had matured enough to handle it. Kai uses it as his primary weapon for the remainder of the series. After the destruction of the Red Shield, he lives with Lewis and David in an orphanage in London, actively hunting chiropterans with Lewis and honing his skills as a soldier in the war against Diva and her minions. With David drowning in alcohol and regret, Kai takes over many of his responsibilities in chiropteran-hunting. Following the tradition of Red Shield, Kai wears a diamond-shaped pendant with a crystal from Riku's remains crested in it.

After Saya kills Diva, he is able to convince her to continue living, and prompts Haji to "disobey" Saya and confess his own love for her. Saya and Kai return to Okinawa and reopen their late father's restaurant. A month later, Saya goes into her hibernation cycle and Kai puts her in the family crypt to sleep. Per his promise to Saya, he raises Diva's daughters as though they were his own. Together, they regularly visit Saya's grave to keep an eye on her while she sleeps.