You think Kotaki is a monster hunter of some kind, or did she just know the real Takahashi before she died
My mind also went to monster hunter, but I also wouldn't be suprised if she were also a monster. Perhaps even a monster hunting monster, Alucard style. That could be the igniting spark to the carnage that will ensue.
(Also in a VN, Kotaki would prolly be the "Good Ending" route for Itou, where they become genuine friends and mayhaps even lovers and she learns that she never needed Takahashi in the first place. Then her grandma sweeps in to deliver a bare-knuckles beatdown of the monsters, happy end.)
(Yes, I might be delusional.)