Kaji Isshū is the second eldest of the seven Isshū siblings and the family’s eldest son. He is ten years old and, along with his brothers and sisters, lives without adult supervision after a tragic accident claims the lives of both parents. Despite his young age, Kaji often appears older than he actually is, a trait shared with his older siblings that reflects the heavy burdens they must carry. His personality is defined by a blend of youthful roughness and a fierce sense of responsibility. While he can sometimes behave in a blunt or gruff manner, this exterior masks a deeply protective nature. He is consistently portrayed as the primary guardian of his brothers and sisters, stepping in to shield them from those who might seek to take advantage of their vulnerable situation. This practical, no-nonsense attitude leads him to act in ways that seem unusually mature for a child, making decisions and taking actions that would be expected of an adult head of household.

Kaji’s primary motivation is the survival and unity of his family. The children are orphaned as a direct result of a kind-hearted plan to give their parents a vacation, a tragic twist that deepens Kaji’s resolve to ensure the family does not fall apart. He refuses to rely on charity, including from a wealthy grandfather, and instead commits to the difficult path of self-reliance. Part-time work and managing the household alongside his older sister, Tsukimi, are central to his daily life. While Tsukimi, the eldest sibling at eleven, struggles with the role of surrogate mother, Kaji complements her by focusing on external threats and physical protection. His role in the story is that of a practical defender and co-provider. He is a key pillar holding the family together, and his actions often drive the narrative of the children’s struggle to be independent. His most important relationships are, of course, with his six siblings, each representing a different day of the week. He holds a particularly significant dynamic with Tsukimi, as they function as the de facto parents. Outside the family, he has a close female friend named Urano, indicating one of the few connections he maintains beyond his immediate duties. Throughout the story, Kaji develops from a grieving child into a resilient and capable young guardian. His growth is not marked by a loss of his rough edges but by a deepening of his commitment and effectiveness in protecting his family. He does not possess supernatural or noteworthy physical abilities beyond his tenacity, courage, and a strong personal drive, such as his expressed dream of one day becoming a baseball player. His true ability is his unwavering will to keep his siblings together against all odds.