As kids, BlackJack was much larger than SpongeBob. However, when he met SpongeBob again, he was only the size of the latter's big toe. BlackJack has a very gruffly voice and is circle-shaped, compared to SpongeBob's square shape. He also has very muscular arms for his small size. On his arms are blue-colored anchor tattoos. He has shiny black hair and dresses up in biker-gang clothing in that he wears a white shirt, red wristbands, blue jeans with a belt buckle like SpongeBob's, and black cowboy-like boots.
As his appearance would suggest, BlackJack is a masculine, brave, rough, and intimidating individual, traits which he loves to exploit when around weaklings like SpongeBob. He does not seem to share a good relationship with sensitive and childlike individuals and bullies them due to their contrasting lack of courage. Despite this, he has a decent bond with his aunt and uncles, showing that he is always not as mean as he seems.
Essentially, he is SpongeBob’s polar opposite.
Abilities and talents
Strength: As evident by his muscles, BlackJack was at one point very strong, able to assault a young SpongeBob and throw him at the wall. However, as he got older he stopped growing and was reduced to the size of SpongeBob's big toe, thus diminishing his strength and causing no harm to his targets, rather tickling SpongeBob when he tried to assault him during the family reunion.
BlackJack went to prison for littering, but was released from prison during the events of "BlackJack."
Role in series
SpongeBob receives a note from his cousin BlackJack, saying that he is going to pay his parents a visit. When he goes to his parents' house, he gets another note from BlackJack, saying he is going to pay their grandmother a visit.
SpongeBob later goes to BlackJack's house and hears his mother screaming from the basement. He goes down there and it is then revealed that SpongeBob's parents are in no danger; they were in fact holding a surprise party to celebrate BlackJack's release from prison, as he renounced his criminal ways of littering.
BlackJack still has the intent of beating up SpongeBob however, who is ready to stand up to himself. As he charges at him, it is revealed that BlackJack is no bigger than SpongeBob's foot and can no longer do any harm whatsoever.
Cousin BlackJack is older and tougher than SpongeBob. 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 was thus cowardly.
BlackJack is known to be a powerhouse. When SpongeBob informs Gary of BlackJack's release from jail, Gary is notably scared.
SpongeBob's grandmother and parents are shown to not be scared of him at all, in fact, they enjoy his company and spend quality time with him.
On the other hand, Uncle Captain Blue SquarePants is very scared to learn that BlackJack was out of prison and was quick to flee to his home after dropping off SpongeBob.