I didn't say plot devices were bad.
I'm just saying, There's nothing to either of their characters other than being a plot device.
Once again, what character did Eto have? How was she different from Rize? They were both batshit insane nearly feral ghouls who's sole purpose was to try to fuck and/or fuck with Kaneki, even more so for Eto.
Just because the story spends more time on them doesn't mean they're developing anything.
btw, Arima is a plot device. I can't call that autist a character.