Jack was a member of the first Royal Flush Gang.
Jack was a metahuman whose body's cell structure allowed him to be entirely elastic. Like the other members of the Royal Flush Gang, he was taken from his family by the U.S. government and educated at Project Cadmus's secret facility in the Arizona desert. The government's original plan was to use him and the other children as human weapons, before they were liberated by the Joker. Free of their captors, and encouraged by their new mentor to use their powers for mayhem, the kids happily became his new henchmen. Jack in particular said that he liked Joker's "style".
Their first caper was to attack the Justice League in Las Vegas in order to stop them from thwarting Joker's bomb plot. Jack enveloped Superman, distracting him from defusing the first bomb, and later fought Batman across several casinos. During the fight, a city's bookie reported that all the odds favored Jack, who was both younger and had the longer reach. However, Batman won the fight, by grabbing one of his elastic arms and using it to entangle the rotors of a helicopter carrying Harley Quinn, knocking them both out of the fight.
It is not clear what happened to Jack after the Joker's plot was foiled.
Powers and abilities
Jack has complete body elasticity. He can stretch his limbs and his body into many shapes and forms.
Jack was voiced by Greg Cipes, who also voices Beast Boy on Teen Titans. Similar to Jack, Beast Boy is the most agile/slippery member of his team. Jack's powers are similar to DC Comics' Elongated Man, Plastic Man and Marvel Comics' Mr. Fantastic.