Keiki Kiryū is a second-year high school student at Momosawa High School with blue-purple hair and matching eyes. He works as an assistant librarian and belongs to the Calligraphy Club. Keiki has an adopted younger sister, Mizuha Kiryū, who is just five months younger. His parents concealed their non-biological relationship throughout his childhood, believing it best for them to grow up as siblings. Kind-hearted and considerate, Keiki genuinely cares for friends and family. He struggles with chronic hero syndrome, often helping others out of concern, like joining the Calligraphy Club after seeing no one accept president Sayuki Tokihara's pamphlets. He actively desires romance, frequently envying couples he observes, but rejects advances from girls with extreme fetishes, prioritizing a "normal" relationship. Unlike typical harem protagonists, he actively investigates romantic clues and confronts admirers about their feelings, though he struggles to fully grasp complex emotional situations involving hidden desires. His defining story begins upon discovering an anonymous love letter paired with white panties in the Calligraphy Club room. Dubbing the sender "Cinderella," he launches an investigation to identify her among several candidates: third-year Calligraphy Club president Sayuki Tokihara, first-year assistant librarian Yuika Koga, classmate Mao Nanjō, and his sister Mizuha. This quest uncovers each girl's hidden fetishes: Sayuki's masochistic desire to be his "pet," Yuika's sadistic wish to dominate him, Mao's fujoshi tendencies focused on pairing him with his best friend Shōma Akiyama, and Ayano Fujimoto's olfactophilia. Koharu Ōtori later blackmails him into helping her pursue Shōma, exploiting her stalker-like voyeurism. Through these interactions, Keiki experiences significant development. Initially rejecting the girls' kinks outright causes conflicts, like callously telling Sayuki he prefers student council work, which deeply hurts her and endangers the club after budget misuse. He later regrets this, working to reconcile with Sayuki and save the club. His perspective shifts after Yuika and Sayuki suppress their fetishes to appeal to him; guided by Mao's counsel, he realizes forcing them to deny their true selves is harmful. This marks a growth toward greater empathy, though he continues seeking a conventionally normal partner. The investigation culminates in the revelation that Mizuha sent the letter and panties, driven by longstanding romantic feelings. Her exhibitionist kink—developed after forgetting underwear during a fifth-grade swim class—explains the included panties. Keiki struggles with her confession due to their sibling relationship but eventually acknowledges his own romantic feelings for her. By the series' conclusion, he commits to a relationship with Mizuha, accepting her exhibitionism as part of her identity. Additional facets include his close friendship with Shōma, whom he frequently consults for advice, and his temporary role in the student council to help repay the Calligraphy Club's debts. His name means "intelligent brightness" and is a pun on "cake," used as a nickname. He prefers café au lait and specific costumes like maid outfits and nurse uniforms. Keiki occasionally displays hypocritical humor, criticizing others' perversions while collecting porn magazines and demonstrating unexpected sadistic tendencies during roleplay with Sayuki.