Also a theory about her ghost design, since the ghosts' forms seem to have something to do with their regrets/grudge like the voyeur ghost having a lot of eyes, ghost who died in a fire being burnt and sooty ect. Her ghost has 3 skulls and bunch of thick strings weighting her down, i think maybe after Touma killed himself, Hayato did the same and those extra 2 skulls represents the guilt of their deaths tying her down lol

I've held it in pretty well the whole time but them showing that the housemaid became a ghost, probably because of her lingering regret that she did not hand the letter to Touma on time, thus causing his death was what finally did me in