I'm going to take a shot in the dark and guess Aya was killed by her old gramps and Erika discovered it, did some kind of ritual to revive/puppet her corpse and nothing came of it, went on with her life only for Aya to get back up and come back to which she was both shocked it's working now but also finds it to be in her best interest since it can possibly help Koto get over everything so Erika can shoot her shot
Erika putting her own happiness after her friends sake, Koto is the only thing that Aya have left, and Koto has being waiting for Aya all those years, i fell sad for Erika.
Erika putting her own happiness after her friends sake, Koto is the only thing that Aya have left, and Koto has being waiting for Aya all those years, i fell sad for Erika.