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 body
It's nice to have the confirmation that "Master" and "Teacher" are in fact two separate people, since the earlier translations used the same word for them (probably because it was "sensei" for both)