He is Yakumo's uncle and father of Nao. He is kind of a father for Yakumo and lives in a Buddhist temple. In order to make Yakumo feel a little bit more comfortable he wears a red adherent lens. Also Isshin was the one who named Yakumo.
Isshin passes away in volume 6 of the light novel. He was in a brain-dead state. Sakakibara stabbed Isshin, however it was actually a non-fatal wound. Sakakibara was the first doctor to treat Isshin, he purposely made it so he could cause Isshin to enter into a brain-dead state so Isshin could donate his heart to his daughter Yoshiko (who was suffering from a serious heart condition) since Isshin was an organ donor. In the end, Yakumo consents to the organ donation. At the end of the volume, they held a funeral for him, with Yakumo telling Isshin's spirit that him and Nao will be okay so it is okay for Isshin to leave too and he does.
