Is the girl in the Old Chateau in Diamond a ghost?

Yes.It is an Easter egg by gamefreak along with the butler ghost in old chateau.

Correct!

Notice that when she walks she does not use her legs. BUM BUM BUM BAH!!! I can't find the ghost butler I WANT TO! :(

the old butler dude is just ahead when you get in. note, though, that when he walks, he doesnt use legs, also, and you don't actually see him go through the wall. nor is there an item. if there was a point in the girl, then that would make this more interesting, or if you chased the butler, he would eventually serve you goods at a cheap price, if only!