It makes me wonder if there is more than one Yuuko, like a split. The Yuuko we see remembers very little before she died, and has always either been lonely, or playful. Even when she's annoyed at Teiichi, the worst she gets is a bit of a teasing pout. Annoyed, never angry.

Perhaps the other spirit is Yuuko's memories and all the anger. Her grudge. Why she would be interested in taking it out on Kirie is beyond me right now, and would only be speculation. But the shrine below the room has got to be a big part of it, and what really happened to Yuuko.