Masao Sato is a five-year-old classmate and friend of Shinnosuke Nohara in the Sunflower Class, known for his timid and sensitive nature that often manifests as fear or anxiety. His shaven head inspires the nickname "Onigiri," used warmly by friends but mockingly by bullies, and occasionally causes comedic mishaps when mistaken for an actual rice ball. A frequent crybaby, he tears up easily over minor incidents or peers' careless remarks, yet displays flashes of courage when motivated by his devotion to crush Ai Suotome. Ai typically treats him dismissively like a pet, triggering his "puppy-mode" response to her whistles—a habit Nene Sakurada once tried to break through training.

Masao enjoys hobbies like collecting phone cards and insects, reading Usuto Yoshii’s manga, and creating his own comics. His cleanliness borders on obsessive-compulsive, driving him to voluntarily tidy chaotic spaces like Shinnosuke’s home. Among friends, he endures light teasing or scolding from Shinnosuke and Nene—such as an incident where a four-leaf clover was destroyed, distressing him—while Kazama and Bo Suzuki comfort him afterward. During Nene’s imaginative play, he reluctantly acts as her "weak husband," fearing her temper.

In "Arashi o Yobu Appare! Sengoku Daikassen" (Movie 10), Masao appears among the core friend group transported to the Sengoku period, participating in line with his established group dynamics. Beyond this, his personal quirks include wearing diapers despite his age and being the Kasukabe Defense Organization’s smallest member. Romantic interests extend beyond Ai to schoolmate Megumi Yamura and an unnamed wealthy girl; a manga-exclusive subplot depicts him developing a crush on an adult woman he observed dressing, omitted in the anime adaptation.