He is light gray and has a light green leaf on his head. He has mahogany-brown pupils and does not smell pleasant, hence his name.
Role in episode
Smelly originates when Squidward falls off his bike and gets stuck in the cement, falls into sewage, and gets a leaf on his head. This makes him look like an animal, hence why SpongeBob and Patrick have no idea that he is indeed Squidward.
SpongeBob and Patrick "rescue" Smelly and Patrick takes him in to live with him. Eventually, the police capture Smelly and put him in the Bikini Bottom Zoo. SpongeBob and Patrick then break into the zoo at night to free Smelly from his cage. They are caught in the act by the police but resist arrest by venturing into the sewer, essentially making the cops lose track of the trio.
At the end of the episode, Smelly is freed to roam with his own kind. Smelly also gets the chance to listen to Kelpy G perform. Shortly after he arrives at the place where his own kind thrives, his true identity is revealed when the various nature elements covering his body fall off.
He is enemies with Gary, who is the only one who knows his true identity.