Since she was young, Umi has been Kousaka Honoka and Minami Kotori's childhood friend, thanks to Honoka inviting her to a game of hide-and-seek. She is also a member of the school's archery club, at which she is quite good. She believes Honoka is bossy and pushy, but also realizes the perks of having an adventurous friend. Among the members of μ's, Umi takes the role of being their physical trainer.

Her father is a martial-arts trainer while her mother teaches traditional Japanese dance, and she is expected to inherit the combined dojo of both. She has a far older sister who has already moved out.