Isshuu Mizuo, the second son in a family whose names follow the days of the week, embodies Wednesday. At nine years old and a fourth-grade prodigy, he earns acclaim for his consistently top-tier academic performance. His sharp intellect fuels an insatiable curiosity for science, manifesting in frequent experiments at school and home—a practice driven by his unwavering goal to become a scientist.

Mizuo’s life fractures when his parents’ sudden death leaves him and his six siblings orphaned. Thrust into survival, the siblings depend on one another, with Mizuo applying his analytical prowess and strategic thinking to navigate their challenges. His role cements him as a pillar of responsibility and teamwork within the family, though his interactions remain framed by his academic dedication.

Visually distinct with brown hair and glasses, Mizuo retains these traits even in the Italian adaptation, which ages him to 14. His narrative remains anchored to these defining attributes, with no expanded arcs or media appearances beyond his core role in the central story.