Duke Yukthomas Féyêdêmuffin, who likes to be called "The Duke," is a character seen in the episode "Summer Job."
Arguably, The Duke could be somebody else. Mr. Krabs says that The Duke is one of his "Regulars" and scolds Mrs. Puff for yelling at him. However, SpongeBob reveals himself as The Duke directly afterwards.
The Duke wears a dark blue suit, black top hat, black shoes and has a moustache. He looks very similar to SpongeBob.