From the spin-off novel Sound! Euphonium: True Stories from the Kitauji High School Concert Band published April 2018, full of short stories
Chapter 3
The chapter begins with a letter from Kaori Nakaseko to Asuka Tanaka the day before their graduation. Kaori notes that one of the only things she regrets was not responding to Asuka's question. The chapter then transitions to partway through Asuka and Kaori's first year. The two discuss the problems in the band, and predict the possibility of the Kitauji concert band schism. Later, Kaori confesses her love for Asuka, though not wholly clear whether platonic or romantic. The chapter transitions back to Kaori's letter, where she continues to profess her feelings for Asuka.
you can read the whole translated chapter here
a part of her letter says:
I want to always be with you, Asuka. I want to be your number one person. That’s why I’m showing my courage by writing this letter. It feels like my writing’s gotten rougher. Usually don’t I write prettier words? I really do.
Asuka. I’m happy I met you, Asuka. Thank you for becoming my friend. And if you’d let me, I ask that you please stay with me.
Asuka. Really, what could I write? I’m only writing your name and that wouldn’t convey my feelings at all. In short, no, it’s weird to say in short, but I’m going to be going to nurses’ school from the spring onward. You said that you would be going to a nearby university. And so, if, and really if, you’d like to, Ah, what’s this? Tears are falling onto my hand. Sorry If you’d like to Asuka, why don’t we live together?
I’ve been fascinated about splitting rooms with you for a while. Of course, I’m not saying the impossible. It’s just that I thought it’d decrease our living expenses. I’ll be waiting your response. Even if you refuse, please give me a response. Or rather, if I don’t hear a response, I’ll go ask you directly.