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.
In Batman Beyond Vol 2 #23: Family Day, Jack Walker was fed up with the job that his sister offered him after he was bailed from jail. He decided to return to the life of crime by forming a new Royal Flush Gang with him as King. Batman asks Ten to help him take down this dangerous and motley crew. Melanie confronts her brother angrily stating that after all the hard work she did to get him out of jail and set him up with a job to go straight but he rejected it all.