Shuuichi Tsukamoto/Relationships

Kumiko Oumae
Kumiko and Shuuichi have a good and friendly relationship. However, in junior high, he called her ugly and told her to never talk to him again. But that changed over the years as they can freely talk with each other about various topics, and they get along so well that they do not talk to each other using the polite form of speech, except for in public places. It is later revealed that Shuuichi has a crush on Kumiko, but she didn't notice it. Shuuichi and Kumiko become quite distant after some events, and have not had a meaningful conversation, except for the one before their competition performance, when Shuuichi kept telling Kumiko it was going to be alright after which they had a fist bump. However, they still see each other as musical peers, and have been using each other for inspiration to improve their playing.

Later in the light novels, Shuuichi tries to grow closer to Kumiko and nearly confesses to her the night before their final concert and 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 which means that Kumiko also likes Shuuichi. In addition to this, she also confessed that she loves him, and they start going out later in the novel. In season 2 of the anime, Shuuichi's role is still a romantic interest, though he does not confess nor go out with Kumiko.

In the novel, Kumiko and Shuuichi are dating.

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 giving a hair clip shaped like a white flower given by Shuuichi 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 is finished next year.

Hazuki Katou
Hazuki had feelings for Shuuichi, but Shuuichi didn't realize it, so he only saw her as a friend, and they develop a good relationship. Later, at the Agata Festival, Hazuki confesses to Shuuchi but is rejected. He tells her that he already has someone else he likes. Hazuki realizes it's Kumiko, and says she will help Shuuichi be together with Kumiko instead. They become quite distant after this event, but he still considers Hazuki as a friend.

Michiru Hakase
Michiru deeply admires Tsukamoto, and proactively converses with him during sectional practices. Eventually, the two grow closer and the two begin hanging out outside of school. Kumiko Oumae mistakes the two as flirting, and becomes slightly jealous. Kanade Hisaishi is also aware of the situation, and filled in Kumiko on her behaviour.