Masashi is friends with Naota and is often seen hanging out with him, Gaku and Ninamori. Masashi also has a job as a delivery boy, and drives a truck around, even though he is only in grade school. His father's truck is later eaten by the terminal core when Mamimi loses control of it. Masashi, like Naota, generally sounds slightly depressed.
He is short for his age, barely taller than Naota. His large, green eyes are always half-closed and have a slant. He has a rather unremarkable nose often represented by only two nostril dots and has plump lips the same color as his light skin. His eyebrows are thin and arched upwards with the same light brown color as his hair. His hair has a wispy texture and is parted down the center, his bangs reaching his ears while the rest can touch his shirt collar. He wears a red vest, pale blue button-up, navy cargo shorts and green ankle-covering socks, as well as brown hiking boots with black detailing. He often carries a black messenger bag that worn close to the body, wrapping from his right hip to back. It has an orange flap broken up by white in the center, a black strap worn under his vest, and an adjustable silver buckle.
Masashi shares the bored, uninterested demeanor of most of Naota's classmates. He's rather quiet and slouches, giving him a tired appearance. Director Yoshiyuki Sadamoto describes him in his Newtype USA section as "a typical selfish brat," though his selfishness is not shown in the anime.
Masashi is close friends with all three of these classmates. Ninamori has the nickname 'Ninamo' from Masashi and Gaku, showing a closer friendship between the three. Naota spends the least amount of time with him due to his chaotic life, but seems to be closer with all of them once Haruko and all the problems she brought with her leaves Earth.
Masashi has little agency in the story besides asking questions and being Naota's friend. He becomes a delivery service for his family's business along with Ninamori and Gaku, but loses his vehicle to the Terminal Core.