She was made of life fibers, normal weapons did no damage to her. Main characters father invents a scissor sword that severs life fibers and hits her in the eye with it. He actually did it to make her think the scissor sword was the secret he was hiding and not the baby (main character) which was the real secret.
The result is that her eye never healed from the wound so she wears that flower patch thing.
She surely is great, and also an uncontested favourite of mine, but Disco Mom definitely takes the cake when we rate the "villany" of KLK characters.
The main reason behind it is that Nui was a sort of an artificial daughter, coming straight from the life fiber womb, so she kinda had the right to be inhuman, downright sociopathic - which even further explains her cruelty, she was simply unable to feel any empathy or compassion, often acting on a whim. Her main intent was having fun, she simply had no conception on what's going too far, and what's not.
Ragyo, on the other hand, was completely human, born as one and living as one, no matter how many life fibers she stitched into her body. She was even able to feel something when she nearly tore Ryuko apart in the last episode, proving she was not sociopathic, unlike Nui. And despite all this she gladly acted like a monster, in more ways than one, willingly bringing doom upon the world and using literally everyone as tools, thus being the most evil and twisted character in the anime.