The Sponge of Mystery is an alter-ego of SpongeBob. He only appears in the episode "Mermaid Man and Barnacle Boy II."
He looks like SpongeBob wearing underwear, a black colored 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.