None of them are evil, they're just mentally ill individuals that can't deal with their emotions now going through a situation that shouldn't have been possible from the start, and that situation directly involves the person that caused all those emotions. For Erika, this is a chance for Koto to get over it, for Koto, this is a chance to get back together with the girl she's been obsessed with for almost a decade, closure.
They'll be harming each other and themselves. It's a tragic situation where none of them can truly "win" because the moment one gets what they want, the other will lose what they want. From how I see it... Aya, freedom, Koto, safety and Erika, love.
The things Erika said, independently of their nature, are true. They are cruel not because of their meaning but because of what it means to accept them in Aya's situation, she herself understands she's got nowhere to go, she's a child in a body the law considers to be that of an adult.
No family left, no other friends that could offer her a place to stay, nobody else other than a boy that understands a facet of her situation. All she's got are two women who were deeply affected by her disappearance and never got over it. The only way she'll get a chance at being independent is with time and even then there's a chance she may just disappear again, if she disappeared once, what guarantees that what brought her back... couldn't just take her away again? I don't think that we'll be finding out so soon what took her away, and I think it's safe to assume things will explode once the cause is made clear. Was it really Erika? Maybe not. If she thinks that she had part in it, does she blame herself? Maybe. Likely Aya didn't sense this (maybe not so faint) animosity Erika has towards her until this chapter.
Erika isn't as mature as she may think she is. She's not above Koto, the difference is that the girl she likes didn't disappear and is instead a different kind of "unattainable", she's there, so close to her reach, but her sight is somewhere else entirely. Instead of moving on, seeking comfort in other things, distancing herself, she kept Koto around while her feelings got more warped over time.
She's throwing a lot of weight on a girl that got weaned off too soon, just like Koto did when her emotions got out of control.
There's no villain, only 3 girls who need to heal before they can hurt each other further beyond repair.
They are being forced now to develop their emotions and figure them out, we can't be assuming one is a "manipulation genius" or "evil" and really, the most tragic of it all is watching a girl having to grow up within months without proper support, crumbs of a safe space or a guarantee she'll be allowed to exist without an expiration date.
Was her return with a goal in mind? What were the terms settled when she disappeared? How will her memories of the day she disappeared affect the current relationship of the three?
And do they really think this will ever work?
Maybe with Koto if she ever learns to be healthier about her emotions and control them because what Aya is doing is the best for her in her current situation as she wants to have a chance at being independent, and she needs to be, most don't want to be in a cage where you have no freedom.
Yes I could be wrong about all of this if later on the author decides that there is conscious malice and manipulation behind their actions, but so far that doesn't make any sense and it's not what is being shown and is closer to a bad faith interpretation of their actions. You can harm others without intending to and it's your responsibility to accept that and make up for it.
Their actions aren't good but that doesn't mean the person behind them is evil by nature
Ideally they'd be mature and give Aya space to process what is happening, but then we wouldn't have most of this work