Kaori is Yuuko's senior in trumpet section. Yuuko is a devoted girl who admires, respects and cares for Kaori deeply to the point of fangirling. 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 relationship gets better as the story goes on. In a conversation with Kumiko during band camp, she acknowledges Reina's talent but not says it directly to Kumiko. In ep.11 Yuuko offers to help Reina which shows that Yuuko is no longer dislikes Reina and now see Reina as friend/junior that she wants to support. At the end of story, She and Reina appears to be on the good term.
Yuuko and Natsuki graduates from the same middle school. Natsuki and Yuuko are seen to fight quite much, but they still care for each other deep down and are on friendly terms and often hang out with each other. These "love-hate relationship" between them are still shown even after they became new president and vice president.
Natsuki once gives Yuuko a "ball", but it was very little but Natsuki revealed that it was filled with her so claimed love.
Mizore and Yuuko graduates from the same middle school. Mizore is another person that Yuuko cares much 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 sloved, Mizore hangs out with Yuuko less often than before but they are still good friend.
Kumiko is Yuuko's junior in the band. Due to what she done in solo trumpet drama, Yuuko is aware that Kumiko dislikes her. However, after they had a long conversation in the night at the band camp, their relationship becomes better. Later, Yuuko is shown to have enough trust on Kumiko to ask her to help her finds Mizore.
Tomoe is the 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 person who Yuuko befriends with. In anime version, their relationship are only shown in the OVA from the first S2 BD where Yuuko and Natsuki try to motivate Riko to kiss with Goto in the festival.