Shu Koibuchi acts as personal assistant to his politician father, Keiichiro Koibuchi, and is the half-brother of Kuranosuke Koibuchi. A childhood trauma profoundly shaped him: he witnessed his father engaged in an affair with Kuranosuke's mother, Lina, inside a closet after one of her performances. This incident instilled a lasting phobia of women, contributing to his socially awkward and reserved nature in romantic situations. Though noted by Kuranosuke as a 30-year-old virgin, Shu demonstrates a capacity for deep emotional investment once he develops feelings. In appearance, Shu is conventionally handsome with short black hair parted to the right, brown eyes, and round glasses. He typically wears formal business suits, consistent with his professional role. His personality blends outward coldness with internal vulnerability, especially concerning romance. After falling for Tsukimi Kurashita—whom he recognizes only in her "after" makeover state, failing to identify her in everyday clothes—he experiences frequent comical daydreams about a shared life. While often passive, he exhibits protective instincts, notably threatening Shoko Inari when she insults Kuranosuke. His social anxiety surfaces in moments requiring emotional vulnerability, causing him to stammer nervously when trying to ask Kuranosuke about Tsukimi. Shu's narrative arc diverges significantly between adaptations. In the anime, he departs Japan with his uncle, encountering Shoko Inari at the airport. Conversely, the manga depicts him as an active supporter of Tsukimi's "Jellyfish" fashion brand. He orchestrates behind-the-scenes efforts to save Amamizukan, securing protest permits and negotiating with Inari to delay eviction. Following Tsukimi's temporary departure to Singapore to protect Amamizukan—facilitated by miscommunication when Shu drops his phone in water—he travels to Italy to acquire a jellyfish-themed engagement ring. His subsequent proposal is rejected as Tsukimi prioritizes her fashion career and personal growth, dissolving their relationship. Key relationships define Shu's development. His bond with family driver Yoshio Hanamori—a same-age friend since school—offers support and comically misguided romantic counsel. His interactions with Shoko Inari involve manipulation: she drugs him, stages compromising photos to fake intimacy, and later develops genuine feelings after he rejects her following a fabricated suicide attempt. With Tsukimi, his romantic interest evolves from attraction to her transformed appearance to admiration for her resilience, though their connection fractures due to physical separation and diverging priorities. His dynamic with Kuranosuke mixes embarrassment over his brother’s cross-dressing with underlying loyalty and protectiveness. Symbolically, Shu embodies themes of "scriptlessness" in rejection dynamics. His blunt marriage proposal contrasts with Tsukimi’s complex emotional processing, highlighting societal pressures around romantic expectations versus personal ambition. His journey reflects gradual growth from trauma-induced isolation toward taking decisive action for others’ well-being.