The Great Manga Detective (GMD) and his Deadbeat Assistant (DA) are discussing the case of "How to Break a Triangle":
DA: So you still think Erica is the guilty party responsible for Aya's disappearance?
GMD: Absolutely! When deciphering fictional cases like these, motive is paramount over evidence. Erica has all of the motive among the three friends. She an obsessive loner with no social skills.
DA: But what was the means?
GMD: I dunno. But it probably wasn't magic or yokai. Maybe Erica's grandfather was an eccentric scientist who dabbled in suspended animation, and invented a stasis capsule.
DA: That's pretty contrived as well! Literal plot device!
GMD: But I have to confess something. Something that has been bothering me for awhile....
DA: What's that?
GMD: I think we have misunderstood Miss Erica's motive.
DA: We have?
GMD: Absolutely. In the conversation between Erica and Aya (and the flashback) a few chapters back, Erica accused Aya of keeping her relationship with Koto secret. But the tone of the question, and indeed the person she was asking were off. If Erica was accusing anyone of keeping a relationship secret (which is suspicious anyway) it would have been Koto. It's not a question you would pose to someone Erica would have considered an outsider or an Intruder, since she and Koto had been friends previous to meeting Aya.
DA: OK.
GMD: So that's when I realized that if Erica's motive was jealousy towards Aya over Koto, which is what we've assumed, then the whole plot makes no sense.
DA: It doesn't?
GMD: Think about it. If Erica wanted Aya out of the picture, then why not just KILL her? Then stage her disappearance or make it look an adult pervert offed her. If she had the resourcefulness to keep Aya to herself for seven years, it would have been much easier to commit murder than go through all of the trouble of kidnapping and keeping her alive, which leads to a messy denouement anyway.
DA: A dead end really. So what's her real motive.
GMD: A really twisted plot development. Erica LOVES Aya, not Koto.
DA: WHAT?! But we assumed...
GMD: Again, think about it! Aya has no family anymore. Her mom's probably dead, and her grandparents are definitely dead. If Erica put her on ice for seven years, the time gap would destroy all of her peer relationships, especially with Koto. She's now exactly the kind of person who nobody would notice being in distress or duress. Predators like Erica are opportunistic, going for easy prey. Plus the theme of this manga is letting go of the past and evolving forward. If Erica is fixated on a fourteen year old girl, whose youth she can preserve despite the march of time, then she is even more of a twisted monster than we've assumed.
DA: Where the story is at now, Aya is staying in Erica's apartment, after breaking the leg of the triangle that keeps her connected with the wider world..... She's in danger.