Maren Sei is a first-year student at Shimizu South High School. He is a Chinese-American who was born in China and was placed for adoption by his biological parents as a result of the country's one-child policy, with the intention of giving him a chance at a better life. He was subsequently adopted by American parents and later came to live in Japan. In keeping with Japanese customs, his family name is Maren and his given name is Sei.

He plays the alto saxophone with considerable skill, a talent inherited from his father, who is also a saxophonist. Despite being officially listed as a member of the Drama Club, he is initially a ghost member who does not participate in their activities. He is scouted by the brass band club and joins them after Haruta, Chika, and Miyoko stage an impromptu play that helps him confront and resolve the inner conflict surrounding his birth and adoption. Following this experience, he formally joins the brass band club.

Maren is often quiet and appears sad much of the time, but his personality brightens noticeably after he becomes an active member of the brass band club. His presence in the club, whether through his skill on the saxophone or his striking appearance, indirectly encourages several other students to join. He is tall and has a refined, Asian-featured appearance with blue eyes and short blue hair.