Shū Koibuchi, a 30-year-old personal assistant to his politician father Keiichiro Koibuchi and half-brother of Kuranosuke Koibuchi, possesses gynophobia rooted in childhood trauma after witnessing his father's affair with Kuranosuke's mother, Lina, during her performance. He is physically portrayed as handsome, featuring short black hair parted to the right, brown eyes, round glasses, and typically wears formal suits. While initially projecting a cold, reserved front, Shū's personality shifts upon developing feelings for Tsukimi Kurashita, prompting vivid daydreams about a future with her. Despite a generally passive nature, he reveals fierce protectiveness, notably threatening real estate developer Shōko Inari for insulting Kuranosuke. His social obtuseness manifests when he repeatedly fails to recognize Tsukimi without her glamorous makeovers during early encounters. His long-standing friendship with family driver Yoshio Hanamori, dating back to school, involves collaborative schemes to pursue Tsukimi, blending both sound and unsound advice. Shū's romantic arc centers entirely on Tsukimi. He falls for her upon seeing her emerge transformed from Kuranosuke's room but struggles to identify her in everyday attire. Their relationship progresses through awkward interactions: Hanamori arranges their first date, Shū confesses his love during a wind-disrupted dinner, and later formalizes courtship via an engagement proposal letter, which Tsukimi accepts. This engagement becomes a strategic plot point when Shū leverages spousal residency protections to legally delay Amamizukan's eviction. Their relationship dissolves while Shū is overseas, coinciding with Tsukimi's deal with fashion executive Kai Fish to save Amamizukan. His interactions with Shōko Inari reveal complexity. Inari drugs him, stages compromising photos for blackmail, and falsely claims they had sex, leaving him deeply traumatized. Later, after he slaps her for faking suicide, she develops genuine feelings. This incident underscores his strict moral boundaries and intense reaction to emotional manipulation. Behind the scenes, Shū supports the Amars' efforts to save their home. He secures a peaceful protest permit, suggests potential allies, and negotiates with Inari to postpone eviction upon learning of the impending property sale. He also acts as the sole confidant for Kuranosuke's mother, maintaining her secrecy. Media adaptations show minor variations. The anime depicts him departing Japan with his uncle and encountering Inari at the airport, bypassing the Jellyfish brand naming event. The live-action drama casts Asuka Kudō in the role.