Koukin Shuuyu, Hakufu Sonsaku's cousin and childhood friend, grew up alongside her as a sibling after her wartime displacement to his family. Born February 23 and aged 16, he attends Nanyo Academy with a frail appearance, featuring dark blue neck-length hair covering his forehead. He typically wears the school uniform: light brown vest, button-up shirt, and black pants. Shy and conflict-averse despite high combat potential, Koukin holds an official C-tier rank while his strategic brilliance and latent abilities approach A-tier. This gap stems from chronic self-doubt and limited battle experience rather than skill deficiency. He harbors enduring romantic feelings for Hakufu, rooted in her childhood kindness toward him. As Zhou Yu's reincarnation, Koukin wields tactical prowess rivaling Koumei Shokatsuryou. His combat style prioritizes precision and ki manipulation over brute force, exemplified when he defeated Chuukou Kyocho by channeling chi through environmental debris. He acts as Hakufu's primary protector, even intercepting her lethal strike against Ukitsu—resulting in his impalement by the cursed sword Hyakuhekitou. His development navigates protective duties and emotional struggles. In *Dragon Destiny*, he abandoned revenge against Koumei for endangering Hakufu after Kan-u challenged him to uphold Hakufu’s values. During the Battle of Red Cliffs, he expanded his strategic role fighting alongside Nanyo and Seito against Kyosho Academy. In *Great Guardians*, he supported Hakufu through disruptions like her unknown sister’s emergence and Saji’s disappearance. While participating in group activities like picnics and beach outings, his combat engagements reduced. He maintains tense interactions with Ukitsu (who feels unrequited affection) and alliances with Nanyo fighters such as Gakushuu and Saji. Koukin survives all events, consistently prioritizing Hakufu’s safety while gradually overcoming battle hesitancy when necessary. His arc balances strategic growth with unwavering loyalty.