Kaori is Yuuko's senior in the trumpet section. Yuuko is a devoted girl who admires, respects and cares for Kaori deeply to the point of getting defensive about her playing the Crescent Moon Dance trumpet solo. Yuuko often asks Kaori if she wants to go home together, and she holds her in high regard. She will get hostile and angry if someone hurts her in any way. The reason Yuuko admires Kaori so much is because Kaori was the person who supported and encouraged her to stay in the band during Kitauji Concert Band Schism last year.
Kaori also likes Yuuko as an underclassmen but does not show it like Yuuko. Yuuko also wants to make Kaori's "wishes" come true before graduating, like playing the solo-part even if it means hurting someone else. Yuuko is also shown being by Kaori's side quite regularly.
Reina is Yuuko's junior in trumpet section. During the first season, Yuuko shows her dislikes toward Reina many times. When Reina was given the trumpet solo, Yuuko became aggressive towards her out of protection for Kaori, whom she felt deserved it more.
However, in the second season, Yuuko and Reina's relationship gets better as the story goes on. In a conversation with Kumiko during band camp, she acknowledges Reina's talent but does not state it outright. In episode 11 , Yuuko offers to help Reina which shows that Yuuko no longer dislikes Reina and now sees Reina as friend/junior that she wants to support. At the end of story, she and Reina appear to be on good terms.
Yuuko and Natsuki graduated from the same middle school. Natsuki and Yuuko are seen fighting frequently, but they still care for each other deep down and are on friendly terms and often hang out with each other. This "love-hate relationship" between them is still shown even after they became the new president and vice president.
Mizore and Yuuko graduated from the same middle school. Mizore is another person that Yuuko greatly cares for. After Nozomi left the band, Yuuko hung out with Mizore so she wouldn't feel lonely, but viewed her as valuable as any other friend. After the conflict between Mizore and Nozomi is solved, Mizore hangs out with Yuuko less often than before, though they are still good friends.
Kumiko is Yuuko's junior in the band. Due to her stance in the trumpet solo drama, Yuuko is aware that Kumiko dislikes her. However, after they had a long conversation during the night at the band camp, their relationship becomes better. Later, Yuuko is shown to have enough trust in Kumiko to ask her to help her find Mizore.
Tomoe is a second year member of trumpet section like Yuuko. Yuuko hangs out with Tomoe from time to time, usually when she doesn't invite Kaori, Natsuki or Mizore to go to somewhere with her.
In novel, Riko is one of the first people who Yuuko befriends. In the anime version, their relationship is only shown in the OVA from the first S2 BD where Yuuko and Natsuki try to motivate Riko to kiss Goto at the festival.