I still believe the human part was stillborn and the specter was all that remained when Kaya was born, which is why Kaya still acts like a child despite being a full specter since the specter grew up fully being Kaya and not having to share a bodySo Kaya is no longer human and will be consumed by the evil spirit of the family that the mother tried to destroy
It just her assumptions that kaya is already a vessel of hiruko but from what we see she's probably a lot more than that and just herself. I think instead hiruko latch her power into kaya, their soul somehow infused and now she's practically reincarnated as kaya?So Kaya is no longer human and will be consumed by the evil spirit of the family that the mother tried to destroy