Masao Sato is Shinnosuke Nohara's classmate and friend. His round face and shaved head resemble a rice ball, earning him the nickname "Onigiri." This appearance occasionally causes comedic mix-ups where Shinnosuke mistakes his head for real food. He measures approximately 106.2 cm tall and weighs about 20 kg.

Naturally timid and sensitive, Masao cries easily when frightened, overwhelmed, or teased by peers like Shinnosuke or Nene Sakurada. However, he exhibits unexpected courage and physical prowess during high-stakes moments, such as bus operation or when deeply motivated. His voice and demeanor shift dramatically in these instances. Though loyal to friends, fear occasionally drives him to self-preserving betrayal.

Within his friend group, Masao faces playful yet hurtful teasing from Shinnosuke and Nene, often provoking tears. Nene forces him into submissive roles during games like "real house" and directs anger at him. He shows puppy-like devotion to Ai Suotome, obeying her commands and whistles willingly out of affection. Attempts to help him resist this dynamic prove ineffective. He resents Nene’s bossiness, casting her as a villain in his self-made manga or mimicking her behavior, though he rarely withstands her dominance long.

His hobbies include collecting phone cards and insects, sometimes sharing crickets with Shinnosuke. He passionately reads manga by Usuto Yoshii and practices drawing comics to become a manga artist. He displays obsessive-compulsive cleanliness, tidying others' homes excessively.

In future scenarios, adult Masao sports longer hair and a more assertive, short-tempered personality. Within the Buriburi Ōkoku no Hihō narrative, he appears as his typical timid kindergartener self reacting to events. Background details include his well-behaved nature and cleanliness at home, plus romantic interests extending beyond peers to an adult woman he observed dressing.