One thing to point out that Shadow Rei's berserk form design wise would possibly be base on Niko's items. Aside of the rabbit, it has earring, belly button gem and yellow frills (like the toy ring) and yellow tassels (sort of like her hair, though maybe there is something else about her design what would point to that). Just something to consider.
OK, sorry for the really late reply, but at lot has happened and I wasn't exactly able to do much of anything here. Let's get back to this.
It's true, the boss seems very based on her items, though what you identified as an earring is just a rip from what I can tell. Generally it's clear it's based on her treasures. It also helps that the boss is named "best friend"... One of the meanings of "Philé", the name Chronos gave to Rei.
