Duke Thomas Feathermuffin, or the Duke as he likes to be called, is an alter-ego of SpongeBob in the episode "Summer Job."
The Duke wears a navy-blue suit, a black top hat with a light purple stripe, black shoes, and has a mustache with a small beard. 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.
Role in episode
When SpongeBob is teaching Mrs. Puff how to deal with manning the register at the Krusty Krab, SpongeBob disguises himself as the Duke. When he orders beans on toast, Mrs. Puff says it is coming right up, but he then responds by saying beans on toast is not on the menu. Mrs. Puff starts shouting at him, and Mr. Krabs comes out and says that the Duke is one of his regulars. Mr. Krabs then tells Mrs. Puff to not let it happen again.
On DVD releases, the closed captions spell his name as "Duke Thomas Féyêdêmuffin."