Kotarou Sato is a 4-5-year-old who relocates to an apartment complex and becomes neighbors with a manga artist named Shin Karino. Additionally, Kotaro joins Shimizu Kindergarten, specifically in the Star Class.

Despite being of a young age, Kotaro displays remarkable independence, possessing the mental acuity of a mature adult.

Frequently seen with a plastic toy sword strapped to his hip, Kotaro emulates a samurai, deeply influenced by his favorite cartoon, Tonosaman.