Kazuki Izumiya is Hirosuke Nakaido's high school classmate, sporting ochre hair and brown eyes. He spends summer vacations delivering flowers for a shop while wearing a straw hat.

Despite maintaining an active dating life with multiple girls that yields no lasting relationships, he actively encourages Hirosuke to confess his feelings to Yasuna Ajimu. Kazuki further masterminds a four-way date involving Hirosuke, himself, and two college girls including Suzue Takemoto.

He remains fiercely protective of his younger sister, Moe Izumiya, quick to disapprove of any boys who approach or flirt with her. Through his encouragement and orchestrated social events, Kazuki serves as a catalyst for Hirosuke's romantic developments.