He first appears in the episode "Squid's Day Off," saying, "I will destroy the Krusty Krab!" as the result of Squidward's paranoia that SpongeBob will damage the Krusty Krab.
In "Something Smells," SpongeBob's breath smells so bad that his alternate-universe counterpart breaks the mirror.
In "Chimps Ahoy," Patrick invented his own universe with Alternate-Universe SpongeBob in it, but SpongeBob thought it was just a mirror.
Alternate-Universe SpongeBob may have been the one Plankton visited in "The Algae's Always Greener."
In "Wet Painters," Alternate-Universe SpongeBob is seen through Mr. Krabs' mirror when SpongeBob tries to figure out how to prevent Mr. Krabs from seeing the permanent paint he and Patrick got on his first dollar, but he leaves when Mr. Krabs comes home, then telling SpongeBob that he's on his own.
He also gets eaten by a monster in "Ghost Host" as part of a long string of jokes told by the Flying Dutchman to scare SpongeBob.
His most recent appearance is in "SpongeBob LongPants," where it is probably the same character that appears in "Wet Painters" and "Chimps Ahoy."
SpongeBob and Alternate-Universe SpongeBob are a lot alike. However, his alternate-universe counterpart acts more dramatic than usual in "The Algae's Always Greener." Contrary to the main-universe SpongeBob, Alternate-Universe SpongeBob appears to have a bit of a rude streak; in "Wet Painters," he refuses to help SpongeBob, and in "Something Smells", he breaks the mirror he's in upon smelling SpongeBob's rancid breath, alluding to suicide.