Master Udon is a character that appeared in the episode "Karate Island" as the main antagonist, and was voiced by the late Pat Morita. Udon is the fish that calls SpongeBob to come to the mysterious Karate Island. He is actually a con man because he captures SpongeBob and tries to make him to 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 that 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 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 "Condo Island", which has been called "Karate Island" and "Clarinet Island".
Near the end of the episode, Sandy says "You're in the soup now, Udon!". Udon is actually named after a type of Japanese noodle soup.