Shuu Koibuchi, the 30-year-old half-brother of Kuranosuke Koibuchi, serves as personal assistant to their prominent politician father. A childhood trauma, witnessing his father's affair with Kuranosuke's mother, instilled a severe phobia of women that deeply impacts his interactions. He possesses a handsome appearance characterized by short black hair parted to the right, brown eyes, and round glasses, typically dressing in formal suits reflecting his professional role. While initially presenting a cold, reserved demeanor, Shuu reveals underlying kindness, passiveness, and comical awkwardness, especially in romantic contexts. Easily manipulated by figures like real estate developer Shōko Inari, he nonetheless exhibits fierce protectiveness when Kuranosuke faces insult. His character arc pivots on developing romantic feelings for Tsukimi Kurashita after seeing her post-makeover by Kuranosuke. He fails to recognize her during subsequent encounters in her everyday, unstyled appearance, unable to reconcile the drastic difference. His courtship is marked by extreme shyness: he stammers discussing Tsukimi and frequently daydreams about a life together. Overcoming anxiety, he confesses his love during a date and formally requests to court her via handwritten letter, gaining her acceptance. Quietly diligent, Shuu plays a critical behind-the-scenes role aiding the residents of Amamizukan apartment, securing protest permits against redevelopment, negotiating delays with Inari to prevent eviction, and seeking allies to protect the tenants, demonstrating empathy despite his own struggles. His interactions with Shōko Inari add complexity; she drugs him, stages compromising photos for blackmail, and falsely claims they had sex, leaving him feeling violated. Later, he slaps her after a faked suicide attempt, an action that inadvertently sparks her genuine feelings. Key relationships include his close friendship with family driver and confidant Yoshio Hanamori, who aids his romantic efforts, and his dynamic with Kuranosuke, blending protectiveness and exasperation. Media adaptations diverge in his storyline. The anime sees him depart Japan with his uncle, encountering Inari at the airport and missing the Jellyfish brand debut climax. The manga extends his involvement: he deduces Tsukimi's identity independently, supports her creative endeavors, and enters a relationship that later dissolves due to overseas commitments and Tsukimi's deal to save Amamizukan. Kuranosuke's mother, Lina, contacts him secretly during this period, with the communication's nature undisclosed.