Since she was young, Umi has been Honoka Kosaka and Kotori Minami's childhood friend, thanks to Honoka inviting her to play a game of hide-and-seek. She is also a member of the school's archery club, which she is quite good at. 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.