Masao Sato (マサオ) (5 years old), Max in the Vitello, Phuuz and Dutch dubs, is one of Shin-chan's friend and classmate from the sunflower class. He is a weak, sensitive boy, and is scared of many things. Even though Masao is known to be spineless, there was once when he betrayed Shin-chan over a pack of candies with the others (Bo, Nene, and Kazama, though only they feel sorry), but eventually comes back to save him after feeling guilty of what he did. He can be easily embarrassed, but believes his and Ai's love can conquer anything. He has been in love with Megumi Yamura. He gets upset when Ai pursues Shin-chan and Shin turns down Ai's advance.
His nickname is "Onigiri" due to his shaven head's resemblance to a rice ball, and there are several moments where Shin-chan accidentally mistook Masao's gray colored head for a rice ball and even tried to eat it. His friends use the nickname as a term of endearment, while the people that bully him use it as a put-down. Shin is one of his great friends, but his thoughtless words often put down Masao and makes him cry. Masao is more like a crybaby. Whenever Nene seems to be angry without her "happiness bunny", she seems to take it out on Masao. She also is angered when Ai treats Masao like a puppy. In one episode she trained Masao to grow immune to Ai's whistles, that lead him into 'puppy-mode'.
Masao likes to collect phone cards and insects and even gave some crickets to Shin. He also likes the manga of Usuto Yoshii, and his hobby is to write manga. He is a good boy at home and likes to maintain cleanliness as shown when "he cleans the mess in Shin-chan's house".
He is known to be a scared cat, and a bit of a cry baby. He is also a "clean freak" bordering on a obsessive–compulsive disorder.
He is always crying on little things and when he is not crying is is always happy. Only rarely he will become angry and when does so he, closes his eyes and shouts. He is the smallest member of the Kasukabe Defense Organization. He is a cry boy and also wears diapers.
First a member of rose house (Migumi), then a new unnamed girl who is rich and comes in limo with a driver. Masao also fell in love with an adult girl in the original series whom he watched while she was dressing
They are very good friends although sometimes Shin Chan makes him cry with his strange ways. Masao sometimes gets angry with Shin Chan because he is liked by Ai Chan.
Kazama and Masao are good friends. Kazama stops him from crying most of the time.
He endures Nene and her real house antics. He always does his best to not make her mad.
They are often together in most situations. They get along pretty well and are best friends.
Masao absolutely adores Ai and would do anything for her. To Ai though, he is somewhat like a pet and is often treated like one. When Masao is feeling feeble, his friends sometimes remind him of Ai, which replenishes his courage in an almost out of character sense.