Okay, time to join in the first chapter speculation, lol.
I'm going for Aya is now a ghost, she died on the night of the festival and has lingering regrets ... I'm also going with:
Koto loves Aya (we know that from the confessions).
Erika loves Koto and was jealous of Eirka/Aya (the annoyance with Aya and that bit where we saw someone - assumed Erika - leaving after seeing Koto and Aya, pg 35) .
Annnnnd ... Aya actually loved Erika but never got chance to express her feelings (and you are right, I have absolutely no basis for this whatsoever! But this is 1st chapter guesswork here).