He is Yakumo's uncle and father of Nao. He is a father-figure for Yakumo and lives in a Buddhist temple. In order to make Yakumo feel more comfortable about his own eyes, he wears red contacts himself. Isshin was also the one who named Yakumo. At the end of the series, Isshin passes away, and Yakumo consents for his organs to be donated to save the life of a young girl.