I think "it's" is the root of the rumor! so charlotte bump into "him"/"her" that night and also the reason why some maid who see the "ghost" has been expelled. They sometimes exchange when they need to. It's also reveal the truth behind "the thin" master in the photo at the room and why the master seldom to see.. i don't know maybe he is sick so that is why they exchange or there is another reason. i like this manga very much, this story is a bit dark and deep, and has an unpredictable plot i adore.
Charlotte's Letter