Meyneth, sometimes called Lady Meyneth by the Machina who worship her is the Soul of the Mechonis and creator of the life that lives within it.
Some time before the start of the game, she fought with Zanza the soul of the Bionis in an attempt to protect herself and her people. Due to the drain this fight had on her, she went to sleep for some time. As she slept one of her followers, Egil decided to take steps to prepare for the inevitable reawakening of the Bionis.
Not agreeing with Egil's revenge-laden yearnings she intrusted her soul to Egil's sister and her devout follower Vanea. From there, it was simply a case of finding a host for Meyneth. That host being Shulk's childhood friend and love interest Fiora.
For a time, Meyneth has complete control of the body of the young, now mechanized Homs. However, after the events of Galahad Fortress, she allows Fiora to regain most control of her body.
Meyneth appears from time to time after this throughout the Mechonis. Often to save Fiora and the others or to try and reason with Egil. She passes away in the core of the Mechonis when she takes a strike to ensure Fiora and the others can live. Fiora carries Meyneth's memories and intentions from that point forward.
Towards the end of both XC1 and XC2, Meyneth's origin is revealed to be in that of a woman named Galea.
Galea was a co-worker of Klaus the man who was split into both Zanza and the Architect. In both games flashbacks we see she's against his plans to try and step closer to the divine. She seems to be much more level-headed in her thoughts on things such as the Conduit and the power it can provide. As far as is confirmed, her last appearance is in this flashback. As for theories, however...
Some believe Galea's body became the Infernal Guldo due to Klaus's messing with Core Crystals due to Rex picking up her ID card after the battle. However, as XC2 reuses many assets in odd places this could be a similar case, as the only other connections this Guldo has to Galea or Meyneth is potentially the black and blue color scheme with orange/red details.