So I have a theory about madame x and oh boy this is a big one.
Anathea, a normal human, meets Vitus(idk what even is he). They dated, got married and Anathea wished for children. Vitus due to his nature of origin couldn't provide her with any so he wished upon the archetype. The archetype vanished but was reborn as 4 different pieces namely Eriena, Maria, Alice and Allen. Things got bad between Anathea and Vitus and so they used Eriena to spy upon each other, which she was not able to handle and had a breakdown. The breakdown caused her to awaken her archetype power. Vitus then seals 3 of his children within the unown dimension and wiped off anathea's memory of them. Sometime later the Gardevoir tricks the policemen and so they killed anathea, thus breaking vitus's spell that was casted upon him by shayda on Anathea's request. Vitus regains his true nature and goes back to doing his 'evil' deeds.
I think everyone knows the story upto this point, everything I have said has already been revealed. The theory starts from here.
Marble mansion is destroyed and they move to chrysolis manor. Maria becomes Marianette. Maria + meloetta = Marianette, the game already plays on a lot of fusion names as it is. she must've fused with meloetta. It was said that the first known sighting of madame x was 7 years before the story starts and that gap is a lot. Like 50 something years alot. I believe during this time Marianette 'wished' for melia or something happened. Melia is Marianette's 'daughter'. Not a real biological daughter but more in the sense of how the 4 children were wished upon. The other thoery i have is that marianette tinkered with garufan 'curses', and Melia is actually Marianette but her own personality too. This is same as how aelita is tealia's daughter and vivian's granddaughter and at the same time she is just a reincarnation of those 2 and has their memories but is a whole new personality.
And so the archetype was reborn within Melia. But it was said that if the archetype is taken out, the person dies. And here comes our 2 brilliant geniuses of the game. Katsu and Anastasia, the mother-daughter duo of rejuvenation inc and close associates of Maria. They built the armor to keep Marianette alive and Marianette now as madame x and anastasia now as nastasia made team xen. At the same place where rejuvenation inc existed.
Now why do i think this theory is credible? For a long time alot of people on this forum have been theorizing relaton between madame x and maria and melia and i think this is the best order of operations to have taken place to put everyone in place.
1) Madame x said that she had a mother who gave her life to protect herself. Anathea gave her life to protect Maria. She is fixated on Melia from the starting of the game. Why? because of the archetype.She placed Melia under Jenner's care so as to take her back under her when the time was right. Even at the top of eclysia pyramid when her mask breaks, someone noted that her hair color was blonde(still need confirmation from devs on this one).
2) For a while I thought that Melia was something like the interceptor but I replayed the game and the dark future part irked me so much regarding my theory. If she was something like the interceptor because of how erin in the den of souls commented she is maria and looks like maria but melia reiterated that she is not. Now if it were to be true, there would be no melanie but there is and she is melia's counterpart. But how does madame x knows she is? Because she knew that she went ahead with the process of creating meleanie in that timeline too because she did it in the og timeline. But this time there was no anastasia to save marianette and so she dies. Leaving melanie with 2 posessions of madame x, the time diamond and yveltal.
Tl;dr - Melia is mariantte's reincanated copy and marianette is madame x