Masashi Satou is Hiroyuki Fujita's classmate and closest friend, their bond formed in early childhood and continuing since kindergarten. He also shared kindergarten and junior high school with Akari Kamigishi. Standing 165cm tall, he has a slim build, pale complexion, short brown hair featuring spiky bangs, and brown, downward-slanting (tareme) eyes. He typically wears standard school attire: a gakuran, jacket, shirt, and trousers.

His personality is peaceful, friendly, humble, and marked by quiet optimism. He often serves as a calming presence during conflicts, using his reserved nature to help resolve tensions. Despite his soft-spoken demeanor, he demonstrates athletic skill as a dedicated member of the school's soccer club, playing football (soccer). These traits contribute to his popularity among peers.

Within his friend group, his role is supportive, frequently seen with Hiroyuki and Akari in everyday school activities. A significant development occurs in "To Heart: Remember My Memories" where he openly confesses his romantic feelings for Akari Kamigishi. This admission introduces new complexity to their relationships, given Akari's established affection for Hiroyuki, placing him within the narrative's romantic dynamics.

Beyond school, he is identified as a younger brother and a pet owner. His involvement in spin-offs, sequels like "To Heart 2," or other expanded media remains minimal beyond a minor appearance.