@anneth_arc She was lost alone and ended up crystalized in the dungeon for long time and I think she was in coma at that time so she lost her senses of doing things
Here's the point for anyone to control demons eye u need a high amount of mana capacity and Rudeus have a huge mana level that's why he was able to master it quickly and he also felt sorry for defeating eris at that moment cuz after that defeat eris will's to fight was broken.