In "Mermaid Man and Barnacle Boy II" and the video games, he is a gray sea slug with red eyes on stalks, a pair of thick, black eyebrows, and a mouth filled with sharp teeth. He wears a green mask and a green suit with a purple collar and a golden triangle on it.
From "The Bad Guy Club for Villains" onwards, he is more realistically designed, and now has a beige body with brown spots and hair sprouting in several places. His teeth are more sharp and jagged, and his eyes are fully red.
The Sinister Slug can use the slime he produces to climb up walls, and can spit slime onto his enemies to trap them.
In the lost episode of The Adventures of Mermaid Man and Barnacle Boy, he was one of the villains to join the B.G.A.T.F.B.C. During the fight between the heroes and the villains, he tries to attack Mermaid Man with his slime, but Mermaid Man dodges the slime and causes the Atomic Flounder to get hit instead. At the lost episode's end, he and the other villains reveal that they were actually together for a book club.
After the Yellow Avenger defeats Man Ray, Squidward reveals that the Sinister Slug has traveled to Jellyfish Fields to cause trouble and prevent the talent show from happening. The Sinister Slug managed to scare Squidward away and made him drop his clarinet while doing so, and Squilliam Fancyson was scared so much that he left for home.
After hearing about the Sinister Slug scaring Squilliam, the Yellow Avenger travels to Jellyfish Fields and uses his strength superpower to get up to where the slug is. After confronting him, the Sinister Slug admits that he can't fight the power that Mermaid Man's belt has, and reluctantly leaves without battling the Yellow Avenger.