The Atomic Flounder is a retired villain in the Mermaid Man and Barnacle Boy series. He can blast people with his atomic breath.
In "Mermaid Man and Barnacle Boy II", he is gray, has a green shirt with checkered pants and a black hat on his head. His super villain form wears a hazmat suit. He appears to be pretty old.
In "The Bad Guy Club for Villains", he has a shark fin, two fish legs, human hands, one eye, an eye stalk on the left side of his head, and has a shark tail. He is shown to be muscular.
A cutout of him appeared in the Mermaid Man and Barnacle Boy TV show as a cutout at the start of "Mermaid Man and Barnacle Boy II." When SpongeBob was with Mermaid Man and Barnacle Boy, they saw the Atomic Flounder who was revealed to be retired. SpongeBob then attacks him on the account that he's a super villain. Enraged, the Atomic Flounder used fire breath to burn Barnacle Boy's head. In "The Bad Guy Club for Villains," him alongside many other villains formed a book club even though Mermaid Man and Barnacle Boy thought they were up to no good but it was revealed at the end that all they did was form a book club.
The Atomic Flounder's atomic ray, which was seen in "Mermaid Man and Barnacle Boy II," is possibly a reference to Godzilla.
He was actually a fish in a lab experiment, but it went haywire and he became the villain he is now.
The Atomic Flounder is the only retired villain and the only one who has aged besides Viro Reganto.