Jack was a member of the third Royal Flush Gang.
Jack's specialty was knives. After Melanie left her family's life of crime, Jack took over as security systems expert, though he clearly wasn't as good as his sister. After the encounter with Batman forced the family to leave him behind, Jack spent a few days in prison before Melanie bailed him out with her restaurant paychecks. He gladly accepted her offer of washing the dishes at her workplace, claiming he was "the best dish washer in juvie hall." Unlike his parents, he seemed to genuinely care about his sister and saw her more as family than any of theirs.
Jack returns in several issues of the Batman Beyond comic books, most notably #23. In the story, "Family Day", he quits the job Melanie gave him and crowns himself as King of the new Royal Flush Gang. Batman and Melanie eventually stop him, with cooperation of the original King and Jack was sent back to prison.