Kosaka is a young man bound for study abroad in London, his departure shadowed by a recent romantic rejection from his former fiancée, Naoko-san. This heartbreak fundamentally shapes his outlook and actions. He champions a logical, analytical view of relationships, frequently arguing against marriage based on love, which he deems illogical and a catalyst for poor decisions—a philosophy born directly from his pain over Naoko-san's rejection. He possesses short black hair, wears glasses, and dresses in a yellow tailored vest over a shirt, a yellow bowtie, and distinctive black sleeve garters on both arms. During his farewell party, his emotional turmoil drives him to excessive drinking, leading to severe intoxication and sickness. This culminates in him vomiting on a collection of erotic prints owned by Toudou-san, resulting in increased debts for Toudou and inadvertently sparking a drinking contest between other characters. His party interactions involve debating love and logic with attendees while visibly wrestling with unresolved feelings. Despite his attempts to rationalize his heartbreak, his susceptibility to alcohol highlights his underlying emotional vulnerability. His relationships remain defined solely by his past engagement to Naoko-san.