Masao Sato is a five-year-old member of the Kasukabe Defense Force, attending the Sunflower Class kindergarten alongside Shinnosuke Nohara and friends. His shaved head resembles an onigiri rice ball, earning him the affectionate nickname "Onigiri" from friends, though bullies use it as a taunt. This feature sometimes leads to comedic moments where Shin-chan mistakes his head for food. Personality-wise, Masao is timid and sensitive, often crying when scared or criticized. He exhibits strong "clean freak" tendencies, bordering on obsessive-compulsive disorder, frequently tidying spaces like Shin-chan's house. Despite his fearful nature, he displays unexpected courage in emergencies, such as driving a bus or defending loved ones. Intense emotions, particularly his infatuation with Ai Suotome, trigger a transformation into a more assertive, physically capable state marked by a rougher voice and heightened athleticism. His hobbies include collecting phone cards and insects, sometimes sharing crickets with Shin-chan. He aspires to be a manga artist, idolizing creator Usuto Yoshii and practicing comic drawing. In group play, he often endures bullying from Nene Sakurada, who assigns him humiliating roles like the "weak husband." His relationships are complex: he adores Ai Suotome but is treated like a pet, responding to her whistles in "puppy-mode." He envies Shin-chan for Ai's affections and occasionally resents Nene's dominance, casting her as a villain in his stories or mimicking her behavior—though he typically loses confrontations with her. Fear can drive him to betray friends, like abandoning Shin-chan over candy, though guilt later compels him to make amends. Across media, Masao experiences nuanced development. In the film *Arashi wo Yobu Jungle*, he and Nene assume married roles in an alternate reality, highlighting their fraught bond. Depictions of his adult persona show a significant shift, featuring longer hair and a short-tempered, tougher demeanor contrasting his childhood timidity. His details include an Aquarius zodiac sign, height of 106.2 cm, weight of 20 kg, and blood type A.