Little is know of Rex Hunter's past, but what is known is that he was once one of Rip Hunter's students. He had great potential, but Rip kicked him out of the Time Masters when he was caught trying to throw a 10 year old Luthor in front of a train and change the past. His real name is Jason Goldstein, but like all great Time Masters he changed it so that no villain could ever truly find him.
Major Story Arcs
52 Pick Up
Rex, still mad at Rip, went to work secretly for the Time Stealers. He would go with the evil Supernova and try to change the past to make themselves the heroes. However, Booster and Rip stopped them and Rex was taken back to Rip's lab where he was tortured. Before Rex could tell Rip anything he was ripped out of the time stream.
The Time Stealers had found out Rex's real name and gone back in time and killed him in his crib and made it as though he had never existed.
It turned out that instead of not existing, Rex's body was turned into pure chronal energy. He could travel through time but he could really effect anything and it was difficult to hold a physical form. So he kidnapped Goldstar, Booster Gold's sister, and takes her to Vanishing point, where its revealed that Rex has found a portal that would make him Time's Ultimate Master. Booster and Goldstar are able to stop him and Skeets is able to make him into a solid form long enough for Booster to stab him.
Being stabbed does not truly kill Rex but his body's energy is dispersed through the time stream to never be seen again.