Koukin Shuuyu, Hakufu Sonsaku's cousin and childhood friend, grew up alongside her as siblings after her father entrusted her to his family. He pledges unwavering service and protection to her, even at mortal risk, while nurturing a deep-rooted romantic affection. As a Nanyo Academy student, he begins with a C-rank fighter status despite latent A-rank potential, hindered by self-doubt and scant battle experience. His strategic intellect surfaces during clashes, most notably when he outwits Chuukou Kyocho at the Battle of Red Cliffs through environmental exploitation and chi manipulation. Following Kouha Kannei's defeat, Koukin rises to become one of Nanyo's "Big Four," cementing his influence within the academy. Specializing in ki mastery over brute force, he neutralizes advanced techniques like Toutotsu's Hakka Daiseishou via precise chi counterbalancing. Though frequently overpowered physically, his tactical ingenuity proves pivotal in combat. His journey includes being abducted by Yoshu fighters and a subsequent confrontation with Seito's Kan-u Unchou. Kan-u dissuades his vengeful pursuit against Hakufu's foes, prompting his return to Nanyo's forces. In the *Great Guardians* arc, he engages with events surrounding Housen Ryofu's enigmatic resurrection and the emergence of a girl claiming to be Hakufu's sister. Later, in the *Western Wolves* OVA, he participates in Nanyo's conflict against Kansai fighters under Himiko. Personality-wise, Koukin exhibits shyness and avoids unnecessary conflict, yet his fierce loyalty to Hakufu fuels his actions. He occasionally displays deadpan snark. Through his development, he gradually conquers his insecurities to become a more resolute protector of Hakufu and Nanyo Academy.