Kōu is a member of the Fūma clan of ninjas and the twin brother of Shōryū. He is among the strongest warriors of the Fūma, possessing the Byakkujin, a special power or technique that few in the world can claim. As a shinobi, he has complete mastery over ninjutsu, and his combat ability is such that he is able to kill one of the Yasha generals and seriously wound another.

In the conflict between the Fūma and the Yasha clans, Kōu is dispatched alongside his fellow Fūma warriors to support Kojirō. He directly challenges Byakko, one of the Yasha generals. The battle ends in a mutual kill, with both Kōu and Byakko dying. After his death, Byakko takes Kōu’s face and uses it to infiltrate the Fūma clan, a deception that is eventually uncovered by Shōryū. Kōu’s twin brother then sends a warning to the Yasha side before also perishing.

Kōu is driven by his loyalty to the Fūma clan and his sense of duty in the long‑standing rivalry with the Yasha. His role in the story is that of a capable but ultimately sacrificial fighter, whose death leads to a significant plot development involving his brother and Byakko’s infiltration. His relationship with Shōryū is the most defining personal connection, as the twins share a bond that becomes crucial to the narrative when his brother detects the impostor. Throughout his brief appearance, Kōu demonstrates the fierce commitment and skill expected of the Fūma elite, and his loss marks a turning point in the war between the two clans.