The one thing I really like about Phos is their Frankenstein amalgamation of a body. It's a great blend of the standard Shonen-style "power-ups" and a visual representation of their personal growth. Phos is a literal example of the concept of the Ship of Theseus.
I find myself likening Lapis Lazuli's head with the forbidden fruit of Eden. Knowledge is what makes us sinful and truly human.
Wait. If Phos is truly a human, then why not just make them pray them selves? All Adamant does is function as a substitute human for sending the dead souls to heaven or whatever. So why not just teach Phos how to pray and make them do it themselves? Granted Phos would end up alone as the last living being alive but whatever. The author already likes torturing them for no apparent reason anyway.