AnimeCrimson1

I think either that Madame X is either Celine,Anathea from another timeline, a living member of the Garufa or Melia/Melaine from said another time line

There are theories for this

Her being Celine is unknown because both are mysterious characters since Celine was only mentioned by Amanda

Her being a alternate counterpart to Anathea is slightly possible because both know Maria and have a hatred towards Vitus but Anathea cares about her husband until she found him of what he actually is and Madame X has hatred towards him for making Maria suffer

In V12 it was revealed that Anathea used to have black hair but was changed to blonde due to the Archetype and Madame X has black hair if you beat Amanda and having red eyes which could assume that the Archetype changes the appearance and personality of the person

Possibly being involved with the Garufa is slightly because she knows about the Jewel of Life and possibly knows the use of Garufa Magic

Her being a alternate counterpart of Melia because Melia was able to go far because of her friends being there for her and it is implied that Madame X had friends but something changed her and they both know the player

Another point I made is slightly to be a counterpart or indeed a version of Nymiera but not possible since Nymiera isn't evil and her descendants Alexandra and Damien are good people