THANK YOU FOR THE TRANSLATION!!
It seems that even though she loses her memories, she's not "mentally" 10 judging from her deduction skills. I'm thinking she must've grown up and something happened which made her suppress her memories with the help of Sayaka.
I say that because in one of the chapters, Sayaka told her "you won't feel like dying every time you wake up" which I interpret that as past Kurumi asking her to supress her memories.
Edit: the correct line from chapter 18 is "You won't feel like dying every time you sleep" on page 21. It really looks like she's sincere so I doubt she's a bad guy.