Tbh, re-reading the earlier chapter, there is a hint about Erika’s wish at the Tanabata festival as an adult 7 years later wishing Aya to come back.
This makes me think Aya’s disappearance may be Erika’s wish at the Tanabata festival 7 years ago as well.
Koto is not the best character in this manga, but some comments have been quite harsh to her. She really loves Aya (maybe not the Aya that she is afraid of changing right now, but the ideal Aya from 7 years ago) - this kinda make the story make sense why Koto is so traumatised with losing Aya to someone else and that she is moving forward too fast trying to try new things. Reflecting this back on Koto’s past experience with Aya’s disappearance and Aya’s thought of “going somewhere” - of course she would not want to let Aya go. Not saying that as an adult, this is the correct behavior to especially a minor like Aya. On the other hand, this is why the character development and plot lines of the manga so interesting.
They are not perfect characters that we will all like or a fan of. But to all of their defenses - the three of them are perfect depiction of what a manga character is and the build-up for all of the mystery is super fascinating to follow every month.
Can’t wait for more!! If you can buy the comics to support sensei, that would be great too!