Koyou Mougai is a twenty-two-year-old river bandit leader who bears the Monkoku star mark on his neck, marking him as one of the Five Divine Warriors fated to clash with the corrupt imperial forces. As the chief of the Uzu-dan water bandits whose territory spans a great river, he cuts an imposing figure with a large, lively frame and a rough, boisterous manner. Beneath that rugged exterior, however, he possesses surprisingly dexterous fingers and a genuine enthusiasm for cooking and makeup, revealing a more intricate and flamboyant side.

His hometown was destroyed by the ambitious general Keirou Hakuhou while he was away fishing, leaving him with deep personal resentment and a hardened outlook on the empire. Despite this, Koyou is capable of sincere compassion. He rescues Taitou, the bearer of the Hagun star, and the archer Hosei after the two are swept helplessly downriver, then offers them shelter on his enormous barge, the Touga. During a devastating flood, he uses the vessel to evacuate stranded civilians, an act that formally aligns him with the heroes opposing Keirou. His initial meeting with the group came through Laila, Taitou’s sister, who actively sought his aid.

Among the Five Divine Warriors, Koyou wields a formidable eight-section staff called Garyuusen, fighting with unrestrained force. His role in the story progresses from that of a self-interested bandit chief to a committed ally whose ship serves as a mobile base and whose brawn proves invaluable in battle. He holds a lingering wariness toward the elderly monk Sonnei, whom he encountered in the past and around whom he feels notably awkward. His chief motivation is revenge against Keirou, yet over time this personal drive blends with a broader dedication to the group’s cause. Throughout his journey, Koyou tempers his impulsive independence with steadfast loyalty to his companions, and his deft hands, whether wielding a weapon or preparing a delicate dish, remain a defining hallmark of a warrior who is far more than a simple brute.