Masao Sato, nicknamed "Onigiri" for his shaven head's rice ball shape, attends Futaba Kindergarten's Sunflower Class as Shinnosuke Nohara's timid and sensitive classmate. He frequently cries when scared or stressed by minor provocations. As the physically smallest member of the Kasukabe Defense Organization, Masao exhibits obsessive-compulsive cleanliness habits, organizing meticulously at home and tidying others' messes. His hobbies encompass collecting phone cards and insects like crickets, which he occasionally shares, creating manga inspired by Usuto Yoshii, and harboring intense romantic interests—first for Megumi Yamura, then developing near-worshipful devotion to Ai, obeying her whistled commands like a puppy. Nene attempts to train him to resist Ai while also bullying him into playing the terrified, submissive husband in her "real house" games, a dynamic echoed in a 2004 film's alternate adult marriage storyline.

Masao shares a close but friction-filled friendship with Shin-chan, whose careless actions often upset him. When distressed by Shin-chan or Nene, he seeks comfort from Kazama and Bo. His anxious personality includes wearing diapers. Across films like *Eiga Crayon Shin-chan: Shinkon Ryokou Hurricane - Ushinawareta Hiroshi* (Movie 27), OVAs, and the 2016–2017 web series spin-off, Masao consistently appears as a core group member in adventures, maintaining his sensitivity and loyalty.