This is Katherine's room, and the most likely place you would find her if you need her. It's a good size, and one of her favourite places to hang out. Be careful though, it is a complete mess, and ballet music is often playing in here.
Katherine has her weapons hidden in here, as well as a stash of ambrosia, though she keeps their whereabouts hidden. There is also a bookcase filled with texts both in Russian and English, and there are other items that belong to her stashed around as well.