Yoshiuo Minamoto is the final antagonist of the anime. He is a handsome and wealthy aristocrat, who is referred as a "noble" in the dub and he had secretly caused the disappearance of several girls who were invited to parties he hosted. He was the employer of Akeno Shiranui where he gave her orders to break up Nagasumi Michishio and Sun Seto so he could have her for himself, though his reasons were based only on finding Sun physically attractive.
- Yoshiuo has three giant and carnivorous eels as pets.
- Even though Yoshiuo claims to be a "mermaid noble", his speaking and mannerisms are similar to a punk (such as referring to Sun Seto as a "babe" or calling Nagasumi Michishio a "drag").
- Yoshiuo's true form is a repulsive and overweight catfish, something that his loyal followers had been unaware of and find hilarious after his true form is revealed upon losing his fight with Nagasumi.