Ceroba, the Lady of the Ruins.
"Formerly stood with her husband on his path for vengeance. The Ex-Empress, in a fit of regret, remorse, and sorrow following an incident at the castle involving the Kindness soul, fled to the ruins to atone for her sins and to redeem herself for her choices that led to what she became. Presently, she lives her days as an aspiring pacifist, looking to find peace, help her fellow monsters... and await the next fallen human."
In short, while she was originally on board with standing by her husband to end humanity, it all changed when she had to witness the first fallen child, the Kindness soul, be killed for their plans. Her motherly instincts overpowered her sense of role and she instantly demanded that they stop at once, sighting that no path should be paved with the blood of one so young.
After a heavy fight of disagreement, heartbreak, and hurtful words, she left and exiled herself in the ruins, looking to try and redeem herself for the atrocity she played a part in.