Keiichirō Koibuchi, a wealthy politician and patriarch of the influential Koibuchi family, possesses significant political connections, notably through his brother-in-law, Saburōta Negishi, who serves as the Prime Minister of Japan. Married, Keiichirō has two sons. His elder son, Shū Koibuchi, is legitimate. His younger son, Kuranosuke Koibuchi, resulted from an extramarital affair with Italian opera diva Lina, a circumstance contributing to familial tensions. Keiichirō maintains a notably strained relationship with Kuranosuke, explicitly disapproving of his son's cross-dressing and rejection of political ambitions; he demands Kuranosuke not appear before him while cross-dressed. In contrast, Shū exhibits a more tolerant attitude towards Kuranosuke, despite finding his brother's hobbies embarrassing. His interactions with other family members receive minimal detail, though his friendship with brother-in-law Saburōta Negishi remains intact despite Keiichirō's infidelity towards Saburōta's sister. Across adaptations, Keiichirō's role varies: he primarily functions as a background figure influencing his sons' storylines in the manga and anime, while the live-action film casts him in an antagonistic role. No significant character development or evolution is depicted across the series, films, or television drama adaptations. His function remains largely consistent as an authoritarian father figure whose political legacy and personal choices impact his sons' lives and motivations.