Mizuo Isshū is the second son of the Isshū family in the anime Tentō Mushi no Uta, one of seven siblings whose names are based on the days of the week. His name, Mizuo, corresponds to water. After the death of their parents, the siblings are forced to live on their own in a simple shack built on their wealthy grandfather's property, and Mizuo takes on a supportive role within the household. He is described as a serious and responsible boy, one who diligently helps his eldest sister, Tsukimi, and eldest brother, Kaji, in their efforts to care for the younger children and maintain their daily life. While he does not stand out with any particular special ability or dramatic personal arc, his consistent dependability and willingness to shoulder household duties make him a steady presence among the brothers. He shares a close bond with all his siblings, especially the older ones who act as surrogate parents. Throughout the series, Mizuo remains a quietly reliable member of the family, contributing to their struggle to stay together and survive through their own hard work and mutual support.