Kumiko Oumae/Relationships

Mamiko Oumae
Mamiko is Kumiko's older sister. She likes to tease her younger sister, and she was also the person who inspired Kumiko to play and join the band. They appear to occasionally experience sibling arguments, but they share a close bond as sisters.

Akiko Oumae
Akiko is Kumiko's mother. Little has been revealed so far in the anime about their relationship.

Kentarou Oumae
He is Kumiko's father.

Reina Kousaka
Kumiko and Reina have a quite complicated relationship. In junior high, Kumiko saw Reina being upset and crying because they did not advance to the nationals despite getting gold. This prompts Kumiko to absent-mindedly ask if she really thought they could have advanced. This offends Reina, thus creating tension in their relationship.

Every time they meet, Kumiko becomes very awkward, but Reina always behaves naturally and does not show any signs of embarrassment or discomfort. After several talks with each other, they resolve their issues and grow gradually closer to the point that they both blush when they see each other. In episode 8, Kumiko invites Reina to go to the Agata Festival with her, albeit unintentionally. However, after they begin conversing and relaxing with each other, they drop the polite form of speech and begin calling each other by first names by the end of the night, signaling a new level of friendship between them.

Now that they are friends again, Kumiko is probably the closest to Reina, since they intimately hug each other and share a strong bond of trust. They both have even said to each other that they love each other. Kumiko also says that she "will always stay by Reina's side" when Reina would compete against Kaori. Kumiko said that she "would never abandon her, even if it would cost her life." This indicates that the two share an amicable friendship that is on higher level from Kumiko's other friendships.

Kumiko seems to show a less-seen side of herself to only Reina and at times, Shuuichi, one that is more fiery, stroppy, and determined, unlike the blandly polite facade she presents to acquaintances at school.

Asuka Tanaka
Asuka is Kumiko's section leader and senpai, and is the one who discovers that Kumiko is a proficient euphonium player and ropes her into joining the bass section. In the first light novel, Asuka is very aware of her senpai status to Kumiko and thus always employs great discretion about her personal feelings when speaking to Kumiko. As the story progresses over the light novels, Asuka grows to care for Kumiko, and eventually invites Kumiko to her house and shares the story of her father with her. On the day of Asuka's graduation, she gives a book of euphonium pieces composed by her father to Kumiko, saying that Kumiko have better use for it now than she will post-graduation.

Shuuichi Tsukamoto
Kumiko and Shuuichi are close childhood friends. However, in junior high, when Kumiko invited Shuuichi to her home to play, he calls her "ugly" and asks that she never speak to him again. Soon after they entered high school however, they end up being on good terms again and use that as an opportunity to freely talk to one another about various topics. They get along so well that they do not speak to one another using the polite forms of speech, except for in public places. It is later revealed that Shuuichi has a crush on Kumiko when he asks her to attend the Agata Festival with him. Kumiko seems unimpressed by his offer, and accidentally ends up asking Reina to join her instead. Shuuichi and Kumiko become very distant after this event and have not had a meaningful conversation since, except for the one before their competition performance, when Shuuichi told Kumiko that things were going to be alright after which they fist bumped each other. However, they still see each other as musical peers and have been using each other for inspiration to improve their playing. Prior to them being distant, she seems to show a less-shown side of herself to only Reina and at times, Shuuichi.

In the light novel, Shuuichi tries to grow closer to Kumiko and nearly confesses to her the night before their final concert. He also gives Kumiko a hair decoration that's shaped like the white flower Taki gave his wife before he proposed. Asuka mentions the hair decoration on Kumiko after her graduation ceremony and asks if it's from her “boyfriend.” Kumiko stumbles and eventually says yes revealing that she is also in love with Shuuichi, and they start going out later in the light novels. In the anime, the plot subverts this in season 2 in which they don't start dating at all.

At the start of Hibike! Euphonium Movie 3: Chikai no Finale, Shuuchi confesses to Kumiko and they started dating. They nearly kissed during the summer festival to which Kumiko interrupts the moment as she became embarrassed. Later in the training camp for the upcoming regional competition, Kumiko decides to break up with Shuuichi by returning the hair clip he gave her as she is overwhelmed thinking about her future. However, she mentions to him that she is willing to date him again after their club activities are finished next year.

Hazuki Katou
Hazuki is one Kumiko's first friends upon entering Kitauji High School, and Hazuki called Kumiko by her first name, which quite shocked Kumiko. Normally, Kumiko doesn't get along with sports-oriented people, but she gets along very well with Hazuki. They are on good terms and also, can talk to each other about various topics and so on.

When Kumiko found about that Hazuki liked Shuuichi, she tried to support them as much as possible, and she even declined Shuuichi's invitation to a festival, so Hazuki could go with him instead. This proves that Kumiko cares for Hazuki a lot, and they are close friends.

Sapphire Kawashima
Sapphire is also one of Kumiko's first friends upon entering Kitauji High School. Hazuki, Kumiko, and Sapphire are almost always seen with each other. Kumiko and Sapphire also get along and they are on very good, friendly and close terms and support each other. Her friends and other people close to her call her, Midori.

Natsuki Nakagawa
Natsuki and Kumiko have a good and friendly relationship, as they both play the same instrument. However, it was later revealed that Natsuki was jealous of Kumiko since she played the euphonium better than her. Kumiko didn't realize this before she saw Natsuki practice very hard and when she didn't pass the audition. However, Natsuki later invited Kumiko to talk with her after the audition announcement. They talked about that and after that, they grew closer to each other.

Noboru Taki
At first, Kumiko thinks that Taki was very strict about practicing and going to the nationals, but she realizes that it is all to help the band. She respects him and sees him as a good teacher.

When Taki cuts Kumiko from a euphonium part in Crescent Moon Dance, he does it as respectfully as he can, without mentioning her name. Later, he offers a second chance to be given to her for the second round of competitio, and reminds her to practice hard.

Taki takes notice when Kumiko begins to practice harder, and, like he does with other students, encourages her. In the second light novel, Taki buys Kumiko and Mizore Yoroizuka each a bottle of sweet green tea when he finds them practicing in the school building late in the evening.

When Kumiko is caught out in a typhoon with a broken umbrella, Taki drives her back to her apartment and then gives her his own black umbrella before dropping her off.

After Taki fails to reach his own goals of having the band secure a gold award at nationals, he comes close to a breakdown in his office, shaking all over as he speaks, and tells Kumiko the full story of his wife, and her last living moments, and the promise he made to her. Kumiko consoles him with a few words. Taki thanks her, seeming to feel slightly better.

Michie Matsumoto
Kumiko, like most of the other students, thinks that Michie is a very strict teacher. However, she also respects her and calls her Michie-sensei even when not speaking to her directly. Michie also seems to know Mamiko and gets along with Akiko, Kumiko's mother.

Masahiro Hashimoto
Hashimoto is friendly to Kumiko, though she is not one of his percussion students. During a firework sparkler party, he sits down next to her in a quiet spot and, concerned, asks about Kousaka Reina's relationship with Noboru Taki. He accidentally lets slip during this conversation that Taki is a widower, and ends up telling Kumiko most of the full story.

Azusa Sasaki
Not so much known about their relationship, but Azusa and Kumiko were friends in junior high, and they later reunited when they were both performing at SunFes.