Could the Maiden of The Mist Be...
The Ruined Kings Wife that he was trying to restore?
Disclaimer: If someone has brought this idea up already or something else like that I apologize sincerely, I can be out of the loop at times
This idea comes to me for 2 big reasons
1
The way she talks about him when Yorick buys the Blade of The Ruined King, praising him and speaking good about him, as if he knew him well, and who better to know the man then his spouse?
2
She was the source of this Black Mist mess to begin with, and in Hecarium's lore especially it has been seen just how much of an a change this mist can cause, so it would be possible for her to have been changed into the being she is now
And furthermore, twisted by the Isles dark powers, as many have been.
This was just my 2 cents on the crazy lore we have in this game
Whatcha guys think?