From the Super Mario Wiki, the Mario encyclopedia
The Bathroom (Japanese: バスルーム Bathroom) is a room in Area Three in Luigi's Mansion. In the room there are Blue Twirlers and Water Elemental Ghosts. This is also where Luigi fights Miss Petunia. After Luigi captures Miss Petunia, he gains the Key to the Billiards Room.
Luigi's Mansion (Nintendo 3DS)