As kids, BlackJack was much larger than SpongeBob. However, when he met SpongeBob again, he was only the size of SpongeBob's big toe. It is likely that he stopped growing when he was young, as evident by SpongeBob saying that he hasn't changed a bit. BlackJack has a very gruffly voice and is circled shaped, compared to SpongeBob's square shape and he also has very muscular arms for his small size. On his arms are blue colored anchor tattoos. He dresses up in Biker-Gang clothing (white shirt, red wristbands, blue jeans with belt buckle like SpongeBob's, and black cowboy-like boots). He has shiny black hair, a pet worm, and lives in a cabin-like house only next door to his Uncle Blue SquarePants' house. BlackJack is known to be tough.
BlackJack went to prison for littering, but was released from prison during the events of "BlackJack."
I can turn into a skyscraper!
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Cousin BlackJack is older and tougher than SpongeBob, and he used to be a bully to him and both of them had a sibling rivalry with each other. BlackJack also calls SpongeBob a "little man", not just because SpongeBob is younger than him, but due to the fact that SpongeBob used to not make any attempt to duel with him, and used to become scared stiff.
BlackJack is known to be a powerhouse. SpongeBob's parents were not scared of him at all. In fact, they find it fun that they play and chat together.
BlackJack speaks similar to Macho Man and Hulk Hogan, possibly a combination of the two. The Macho Man was famous for his quote, "Oh, yeah!"
It is unknown how or why BlackJack has stopped growing.