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.
Natsuki always addresses Asuka in an overly respectful way that at first appears to be sarcastic, but is later shown to be sincere and heartfelt beneath the humor. Natsuki and Asuka's dynamic is greatly similar to the relationship between Kumiko Oumae and Reina Kousaka, in that Asuka's passion and drive inspires Natsuki to work harder than she thought she ever could.
When Asuka is forced to quit the band before the final level of competition, she appoints Natsuki as her successor, telling her that it is her turn to lead. Natsuki, however, firmly declines and informs Asuka resolutely that she and the section will wait faithfully for Asuka's return.
Asuka and Haruka share a good relationship as respectively the vice president and president of the band. Asuka is often able to help Haruka when the latter's low self-esteem becomes a problem.
Despite this, Haruka always struggles with the feeling that she was only elected as the band president because Asuka turned it down, and she always feels second to Asuka. In addition, she also feels slightly resentful that Asuka pushed the burden of being the president of a mediocre ensemble on her. However, Haruka's good nature allows her to maintain a friendship with Asuka above the politics of the band.
Taki respects Asuka as a musician and senior student, and Asuka is onboard with his driven methods, especially after she switches her mindset in the second light novel from personal improvement to focusing on nationals.
In the third light novel, Asuka's mother forces Asuka to meet with her and Taki in Taki's office. Asuka's mother demands that Taki allow Asuka to quit the band, but Taki sees the downcast expression Asuka wears and tells Asuka's mother that he will not do anything that is against his students' wills. Furious, Asuka's mother drags Asuka out into the hallway and tries to force her to ask Taki on her own to quit. Realizing that Taki will not let her be removed from the band as long as she does not ask personally, Asuka refuses. Her mother slaps her so hard that she falls to the floor.
When the vice principal decides to accept Asuka's resignation letter on Taki's behalf, Taki does not find out until Yuuko informs him. He leaves rehearsal quickly and attempts to stop the letter from being processed, but he is too late.
Though she does miss a few days of school, Asuka eventually returns to the band for the competition.
Asuka's father, Shindo Masakazu, is one of the best euphonium players in the world, and is also a respected figure in the concert band world. The silver euphonium that Asuka plays belonged to him. That instrument, along with a book of euphonium music composed by her father, are the only two reminders of her father that Asuka possesses, because her mother completely cut off all contact with Masakazu since they divorced when Asuka was very young.
Asuka has always felt her father's musical talent within her, and naturally took to playing the euphonium. Her mother opposed Asuka's love for music her whole life, but tolerated it as long as Asuka kept up her perfect grades, and as long as Asuka never played in a band that would attend the national band competition, which Asuka later finds out that Masakazu is known to judge at.
When Asuka and the Kitauji Concert Band do make it to the nationals, Masakazu gives the band the lowest rating possible, but adds a personal note on the scoring sheet that the principal euphonium player is very talented. Noboru Taki relays the message to Asuka, unaware that Masakazu is Asuka's father, and Asuka smiles when she hears this. It is described in the light novels that this is the first genuine smile she has worn in a long time.