I've read somewhere that Jeanne is not one person,but 5 different people who are related and they share the same name....so since she doesn't have any True Rune as Lemmy said,and she appeared in S4 which is the oldest one in chronological order....that theory being 5 different people who are related,is the most reliable.
Yes, but the dog koroku in S3 and eresh in S5 know something else about her.
Look how Jeane told the dog to not tell anyone about her true self or it might scare people, why with just the resemblance among family members could scare anyone?
Eresh always talk of herself as if she's just someone who is temporily residing in that body we see and she call it her "vessal" then refers to Jeane as comrade so i'm not sure if that's ok to say Eresh and Jeane both employ the same method to make this eye deceiving trick.
So im guessing Jeane's true form is actually a spirit/a fairy/ or even a good will succubus( the charm rune gives me this thought)