What my theory is that he is born with the book containing all the diffrent kinds of demons which manifest in diffrent people. He then helps them overcome whatever trauma or problem they have before they completely turn into demons. For example in the second story, Beelzebub the King of gluttony turned them into maggots and she turned into the fly queen. But she managed to change back into a human when she realises she needs to eat as well. Hence the last picture she is eating with him instead of him eating by himself with her watching.
What my theory is that he is born with the book containing all the diffrent kinds of demons which manifest in diffrent people. He then helps them overcome whatever trauma or problem they have before they completely turn into demons. For example in the second story, Beelzebub the King of gluttony turned them into maggots and she turned into the fly queen. But she managed to change back into a human when she realises she needs to eat as well. Hence the last picture she is eating with him instead of him eating by himself with her watching.