Duke Yukthomas Féyêdêmuffin (or The Duke as he likes to be called), is a character seen in the episode, "Summer Job." The Duke ordered beans on toast which is not on the menu.
The Duke wears a navy blue suit, black top hat with a light purple stripe, black shoes and has a mustache with a small beard. He looks very similar to SpongeBob. 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.
When SpongeBob is teaching Mrs. Puff how to deal with manning the register at the Krusty Krab, SpongeBob disguises himself as The Duke, and when he goes and orders he orders beans on toast, Mrs. Puff says it's coming right up but he, (SpongeBob), then says there isn't beans on toast in the menu, Mrs. Puff starts shouting at him and Mr. Krabs comes out and says that The Duke is one of his regulars, and tells Mrs. Puff not to let it happen again.