In the true ending the kids together become a Bodhisattva.
Specifically, they become their universe's version of Guan Yin, the Bodhisattva of Mercy. Venus brings the thousand eyes for seeing everyone who suffers, Jupiter the thousand arms to reach out to everyone who needs help, and Neptune the desire once they've reached those suffering to get them to drag themselves out of their hell (though she's much more forceful about it than our world's version). This does have the unfortunate side effect of conflating the Devil and Buddhism, but given the specific type of Christianity at play in the game world it probably very much does view other religions as the Devil trying to sway folks from God.The devil is a Hell Imperator.
They regularly come to the camp to find a new Noble of Deviation to elevate, hence the rite; in all but the Golden Ending, they fail and the chosen girl just becomes a normal demon, horrific and yet loved, but their acceptance of him in their lives otherwise allows him to chose ''all three' as Nobles, resulting in their much more angelic (aka Fallen Angel)-esque forms.