Masao Sato is a five-year-old boy in Futaba Kindergarten's sunflower class and the smallest member of the Kasukabe Defense Organization. His shaven head, resembling a rice ball, earns him the nickname "Onigiri"—used affectionately by friends but mockingly by bullies. This trait sometimes causes comedic mishaps when others mistake his head for an actual rice ball. Highly sensitive and timid, he often cries when frightened or embarrassed. He harbors an unwavering belief in love, particularly his adoration for classmate Ai, who treats him like a pet. Thoughts of Ai or love occasionally ignite uncharacteristic courage in him. His vulnerability makes him prone to teasing from friends Shin-chan and Nene; Shin-chan's careless remarks or Nene's frustration (especially without her "happiness bunny" stress reliever) provoke tears, though their intentions aren't malicious. Nene once trained him to resist Ai's whistles that triggered his "puppy-mode" behavior. Masao collects phone cards and insects, sharing crickets with Shin-chan. He reads manga by Usuto Yoshii and creates manga as a hobby. Domestically, he is well-behaved and fixates on cleanliness, sometimes tidying messes at Shin-chan's house to near-obsessive degrees. He wears diapers despite his age. Romantically, he developed feelings for Megumi Yamura, a wealthy rose class classmate who arrives by limousine, and in the manga, an adult woman he observed dressing. His friendship with Shin-chan involves frequent play, though Shin-chan's antics annoy him and he envies Ai's crush on Shin-chan. He fears Nene but joins her "real house" games due to intimidation; their closeness surfaces in scenarios like their husband-wife dynamic in a 2004 movie. Kazama and Bo comfort him when upset, with Bo being a steadfast friend despite limited solo interactions. In *Crayon Shin-chan: Roar! Kasukabe Animal Kingdom*, Masao visits Shin-chan's home after his parents turn into animals. He consumes a partially prepared animalization drink, transforming into a rabbit with a human head. Joining the group's mission, he follows antagonist Bunbetsu to an underground forest base and uses animal abilities to defeat henchmen. Once, he betrayed Shin-chan for candy with Bo, Nene, and Kazama, but guilt compelled him to return and assist Shin-chan. Despite his fearful nature, he demonstrates loyalty to friends in key moments.