Master Udon is the main antagonist seen in the episode: Karate Island. 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 from getting to him. It is revealed that at the end of the episode, he has extremely large muscles and loses to Sandy and falls out of the condo window, landing in the water and possibly dying until…He returns at the very end of the episode (as a pun joke).
 Master Udon's henchmen
- At the end of the episode, Udon is back to normal after falling from a high distance and greeting Squidward to become "King of Clarinets".
- Near the end of the episode, Sandy says "You're in the soup now, Udon!" Udon is actually named after a type of noodle soup served in Japan.
- Works on the 4th floor of the 4 floors of fear
- Udon is the last role Pat Morita had before he died just after he finished the episode. The episode is dedicated in his memory. Because of this, Udon may not appear again.
- He is Voiced by Pat Morita.