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 the team's security systems expert, but he clearly wasn't as good at it as his sister was. 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. Melanie explains to her brother that she was able to get him a job as a dishwasher at the restaurant she worked at, which he gladly accepted, claiming "I was the best dishwasher in juvie hall." Unlike their parents, Jack seems to genuinely love and care about his sister, and clearly 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 previous King and Jack was sent back to prison.