Another insei friendly with Hikaru, Isumi is the eldest in Hikaru's peer group, turning 18 during the series. He appears to be doing very well in the first pro exam shown in the manga, going undefeated until his match with Hikaru. During that match, rumors about Hikaru's incredible strength add to the pressure of time constraints, and he misplaces a stone; his attempt to move the stone after letting go of it prompts his honorable resignation before Hikaru can accuse him of cheating. The match weighs on his mind and he loses his next two games and the chance to become a pro. He later goes to study Go in China in one of the few character arcs in the series that does not center on Hikaru. There, he learns to control his emotions and passes the next year's pro exam without a single loss. Isumi also unknowingly helps Hikaru get over his grief for Sai when their practice game makes Hikaru realize that by playing, he can find Sai within his moves, the Go that Sai passed on to Hikaru. Isumi's birthday is April 18.