Perhaps it's a key for opening any lock. Nitori must've designed it herself so that's why she's always carrying it around.
some kind of a universal key then?
Clothing DesignHuh? That's strange...There it is!!!Maybe I dropped it when I was outside...Ah!!Grrr...I can't find the key for opening the grimoire.This oneYoukai MountainI'm sure I was taking a walk here yesterday...hmm?
