Isshuu Kaji serves as the second child and eldest son in his family, embodying Tuesday within their weekday-naming tradition. At ten years old and in fifth grade, he shoulders substantial domestic duties alongside his older sister Tsukimi following their parents' death. Kaji adopts a paternal stance, often disciplining younger siblings to enforce household rules and preserve family stability. Academically, he underperforms compared to Tsukimi, drawing unconventional punishments from his teacher Mr. Ban—such as standing while holding water-filled buckets, evoking ancient Greek methods. His manga storyline depicts aspirations to become a professional baseball player, while the anime adaptation shifts this ambition toward boxing. Kaji navigates these challenges with directness and unwavering dedication to supporting his siblings in their parents' absence.