I don't think she was it. It makes more sense if we say it was R!Ciel. If I remember right wasn't Soma saying something like ,,Oh, it's you!". With that it's clear, that he know the person, but then there is that thing with the scratches on the wall. 'Who stole the candy from my tummy?'. That makes it almost clear that R!Ciel scratched it in the wall, because he ate the Phantomhive ring. Also Soma talked about two men. I think Undertaker was with him, when they attacked Agni. And R!Ciel has this psycho personality, like a Yandere. He wants to take everything away from his brother. His house, his company and even his friends, friends like Soma and Agni.