This is my theory, far-fetched maybe but still..
People become fayth when their sould are extracted by Yunalesca, right..? What if their sould are restricted in some way when they are in the body. When they are removed from the body they change to reflect the persons feelings etc.
For example, Seymour's mother in the Anime Fayth. She feels pain and guilt because of what Seymour became. this is reflected in the Aeon, Anima's apperance.