Koto has always sounded immature to me. The author made it conspicuous because she still calls Aya and Erika with the -chan suffix, even as an adult.
Erika calls them "Aya" and "Koto".
Aya calls them "Erika" and "Koto"
Only Koto adds the childish suffix.
Koto never grew up. Her mental universe stopped 7 years ago. She tried (by dating), but failed.
Also, the reasons Aya and Erika love Koto are different, but explainable.
Koto and Erika are childhood friends. So, Erika has always known that Koto is someone actually affectionate, kind, considerate, doesn't judge her bad character and will stick by her side.
Aya has abandonment and trust issues that she hides behind a tough facade. Her pillars of support, her grandparents, were eventually gone too and yet, she toughened up and forced herself to move forward. I can't see how anyone could hate Aya. At 14 yo, she's admirable, even if a bit naive concerning the grown up Koto and Erika. She's fragile emotionally, but she didn't let that stop her and bottled it all up.
She loved Koto because Koto showed signs of not letting go, even after not having an answer to her confession and Aya believed (and still believes deep down?) that Koto would never abandon her.
But that doesn't keep her from breaking up and moving forward so she can be level with Koto and not depend on anyone. It's been her own obsession for a while (even before the disappearance): to be self-reliant and not depend on others, whether emotionally or financially.
So, if Koto and Erika get together, I think Aya would be hurt, but would still move on. Because she's stronger than both of them.
Now, the supernatural makes a comeback and I'm curious which direction it's going to take the story.