The Sponge of Mystery is an alter-ego of SpongeBob SquarePants who appears in the episode "Mermaid Man and Barnacle Boy II" and the mobile game SpongeBob's Game Frenzy.
He resembles SpongeBob wearing backwards underwear, a black mask, and a pair of dark green gloves.
Role in episode
SpongeBob guises under this alter-ego while on afternoon patrol with Mermaid Man and Barnacle Boy.
Later in the episode, while Mermaid Man and Barnacle Boy are trapped inside the Dirty Bubble, the Sponge of Mystery asks for his autograph and ends up popping him with his pencil in the process.