So, with Dr. Clef being her father and the letter mentioning Eve, would that make the succubus’ mother Lilith? That would actually make a lot of sense. She is so attractive because she’s the daughter of the perfect woman (well, one of the two) and she can’t wear clothing because they had no need in the Garden of Eden.
In the Jewish version of the Garden of Eden, Lilith was the first wife of Adam. However, she wasn’t obedient enough for Adam’s taste, so God threw her out of the garden and she became the mother of all demons.