Shoko

The Castle Ghost appears in "The Creeps" when Finn is the last one 'alive' and hides in a room in the Castle with multiple bookshelves. The Castle Ghost appears, and Finn breaks through the wall to the next room. Everything in the episode was a prank, except for the Castle Ghost, who Jake had no idea about when Finn asked him if he had set up that as well.

Appearance
The Castle Ghost has light greyish-blue skin with long green seaweed-like hair. It is implied that she has the ability to make objects float, where as when she appeared multiple objects were floating around her in mid-air.

Trivia

 * It is possible that the ghost is one of the women in the paintings around the Castle.