For me it was Megumi.
Sitting there crushing over her pictures of Mimura,looking so sweet and innocent.
Even after Mitsuko finds her,she's still acting sweet and innocent instead of standoffish.To me she comes across as the most innocent in the film,especially when she begs Mitsuko to stop coming after her.
That's why her death seems the most saddest in the film.
It's also the most gruesome one.