Kōkin Shūyu, a second-year Nanyo Academy student and male cousin of Hakufu Sonsaku, rises to join the school’s elite Big Four following Kouha Kannei’s defeat. Beyond kinship, he actively guides Hakufu’s growth, pivotal during a crisis where she saves him by drawing malevolent chi from his body through unorthodox means—intimate physical contact that transfers the corrupting energy into her own. This sacrifice brands her with a haunting skull-shaped mark, a stark reminder of the darkness now rooted within her. Having once shared a household with Hakufu and her mother, Goei, before enrolling at Nanyo, Kōkin’s past remains veiled in mystery. His ascension to the Big Four spotlights his combat prowess, cementing his standing within the academy’s power structure. While his personal motivations stay unexplored, his influence persists through Hakufu’s trials and the shifting alliances among Nanyo’s leaders.