Butler just appears in the episode Porous Pockets. He works for SpongeBob. When SpongeBob was rich, SpongeBob made him stand at the front door to check what people were aloud to come into SpongeBob's House and which people weren't aloud in. Not much is known about him.
The Butler is a green fish with black hair. He wears a blue suit, a white shirt, blue trousers, a red and golden belt, black shoes, and a red bow.
(Patrick knocks on SpongeBob’s new mansion)
Patrick: Uh…I’m here to see SpongeBob.
Butler: And are you on the guest list, sir?
Patrick: Well, I don’t think so. Normally, I...
Butler: Well, then, I’m afraid I can’t let you in, sir. Good day, sir.
Patrick: Good day. Uh, your shoe’s untied.
Butler: Hmm, right-o, it is. (Patrick walks inside) Thank you, sir. (Notices that Patrick is gone)