Background:

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.

Umi has a far older sister who has already moved out.[3] Her father is a martial-arts trainer while her mother teaches traditional Japanese dance; as a result, Umi is expected to inherit the combined dojo of both. As the heiress to her lineage's legacy, she is burdened with the responsibility of carrying on her family's traditional Japanese practices and techniques. She also is highly skilled in kyūdō (弓道), or the Japanese martial art of archery. Predictably, Umi's traditional upbringing is reflected in her personality and behavior.

Personality:

Umi has the qualities of a yamato nadeshiko, or a proper and traditional Japanese lady. As such, she is generally shy when having to interact with unfamiliar people. Even with those she is comfortable with, she seems to act reserved by default unless provoked. She is highly intelligent and a very logical solution-seeker. Above all, she is extremely disciplined--nearly to a fault-- and as a result can often be hard on others. This is exemplified in Season 2 Episode 7 when she forces Honoka and Hanayo to do long training hours after they gain a bit of weight. However, this is never motivated by feelings of malice or dislike aimed at her friends. Umi is often overly strict towards her friends, but is the most unforgiving with/hardest on herself. She despises laziness, shortcuts, and/or cheating; she instead places a lot of value in hard work and perseverance.

As self-destructive as this may be, these qualities combined with Umi's strong will makes her well-suited to leadership, which she often utilizes by being one of the main organizers of logistical or business-related action items for µ's. Where her leadership skills lack in sociability, she makes up in organizational excellence. This, in tandem with Umi's inclination to stay in the background when she can, makes Umi the perfect second-in-command to Honoka's leadership in µ's. In this manner, the two operate perfectly as the primary leaders in µ's. Honoka establishes and maintains µ's core of friendship and love for both members and fans alike through her ability to unify, inspire, and motivate. Umi facilitates the less enjoyable, yet arguably more vital, aspects of their idol work.

The foil between Honoka and Umi's personalities affect areas outside of leading µ's. Honoka is more of a happy-go-lucky, modern day schoolgirl, which lead to her impulsive goal of becoming a school idol. Due to Umi's traditional values of proper ladylike behavior, Umi resisted Honoka's pleas for Umi to join her. Umi's strict moral compass made nearly all aspects of being an idol highly inappropriate and embarrassing for her. However, she is shown at times to entertain intrusive thoughts in which she acts entirely contrary to her code of proper behavior. One example of this is when in the middle of practicing archery, she imagines herself as a cheery and energetic idol saying (what will later become) her signature catchphrase "Love arrow, shoot!" (ラブアローシュート); Umi is greatly flustered and ashamed of even having such a thought. When she does eventually agree to becoming a school idol, she is at first uncomfortable with wearing short skirts and opts for pant shorts. However, she learns to get used to skirts as time goes on.

Despite her overall calm and collected personality, there are a few exceptions. In Season 1 Episode 10, the rest of µ's learns (the hard way) that Umi gets very grumpy and angry if she is woken up in the middle of sleeping. Season 2 Episode 5 reveals that Umi's moral aversion to dishonesty is also actually just a literal inability to lie: when playing Old Maid, Umi is painfully unable to keep a poker face. Any emotion she feels is immediately obvious by her facial expressions. In this way, Umi continually loses every game, which frustrates her, because she is entirely unaware of this behavior. Her persistence and determination causes her to repeatedly ask to play another game, but her unawareness of her emotive reactions does not relent.

Ultimately, although Umi is rather strict, especially towards Honoka, she is a genuinely kind person and a very loyal friend who does the best she can to help her friends. She's also well-mannered and always speaks politely, using "です" (desu), "ます" (masu), and other polite forms of speaking in her speech.

Clubs and Hobbies:

As the daughter of an Iemoto (家元, Head of Music & Arts School) mother and a Shihan (師範, Martial Arts Inspector) father, Umi excels in Kendo, Nagauta, archery, calligraphy, and traditional dance. Starting in the second season, she becomes Otonokizaka High School's new student-council vice president, taking over the role from Nozomi Tojo. She likes reading as well.

Umi becomes the group's main lyricist, and writes the song "START:DASH!!", along with most of the rest of their songs. She also trained the members of the group before Eli Ayase took over the role. She either created the dance steps for START:DASH!! or aided Kotori in coming up with the choreography.

Appearance:

Umi has a slim build. She has long dark blue hair that she splits into three bangs and hazel eyes.