Koyō is a 22-year-old water bandit chief leading a group known as the Whirlpool Gang, who operate along a major river. He is one of the Five Divine Warriors, marked by the celestial star Monkoku on his neck, and wields a segmented weapon called the Garyūsen, similar to an eight-section staff. Physically large and given to a loud, coarse manner, Koyō surprises others with his unexpectedly dexterous hands and hobbies that include cooking and applying makeup. His motivation stems from a personal tragedy: Keirou’s forces destroyed his home village while he was away fishing, leaving him with a deep resentment toward the imperial regime. Koyō first crosses paths with the main party when he rescues Taitou and Hosei after they are swept into his river territory, and he subsequently takes them aboard his enormous barge, the Touga, which becomes a mobile sanctuary and a means to evacuate civilians during a devastating flood. He readily joins the group in their wider struggle against the empire, providing not only combat support but also a refuge for those displaced by the conflict. While generally easygoing and willing to help, Koyō carries an uneasy history with the monk Sonnei, whose presence makes him uncomfortable, hinting at unresolved tension. As the journey unfolds, he grows from a brash, independent outlaw into a reliable and compassionate companion, balancing his rough exterior with a nurturing side and a fierce loyalty to his newfound allies.