Master Udon is a character that only appeared in the episode "Karate Island," and was voiced by the late Pat Morita, who was known for his appearances in the Karate Kid movie series. Udon is the fish that calls SpongeBob to come to the mysterious Karate Island. He is revealed to be a con man because he captures SpongeBob and attempts to make him buy real estate. He has quite a few recruits and orders them to stop Sandy Cheeks from getting to him (The Tickler, Lip Service, and Filthy Phil).
It is revealed at the end of the episode that he has extremely large muscles when he fights Sandy. Though quite muscular, he seems to have a low fighting capacity, as when he fights, he uses sloppy punches and forward slaps, though he actually is able to go toe to toe with Sandy. He is defeated by her and falls out of the condo window, landing in the goo/water. He is shown to have survived at the very end of the episode after SpongeBob and Sandy leave, greeting Squidward to declare him as "King of Clarinets."
He is the owner of the "Condo Island," which has been called "Karate Island" and "Clarinet Island."
Near the end of the episode, when Udon falls into some water, Sandy says "You're in the soup now, Udon!" Udon is actually named after a type of noodle soup served in Asia.