On her 15th birthday, Sentinels controlled by Onslaught destroyed her apartment, killing her parents and friends, and for weeks Hallie hid in the ruins of the city with several children she rescued. When Hallie went to seek help after Onslaught's defeat, she and all the children she was protecting were kidnapped by the mercenary group the Rat Pack and experimented on by the Rat Pack's employer, Arnim Zola. The other children died or were horribly mutated, but Hallie became faster, stronger, and gained the ability to throw bio-electric punches. She escaped and attempted to reach the Fantastic Four.
Having been isolated between the destruction and the experiments, she had not known that many superheroes, including the Fantastic Four and most of the Avengers, had been whisked to a Pocket Universe created by Franklin Richards, the son of Mister Fantastic and the Invisible Woman of the Fantastic Four. The world at large presumed the heroes dead. When Hallie reached Four Freedom Plaza, the Fantastic Four's old headquarters, she encountered the Thunderbolts, a new super team who were in fact the Masters of Evil posing as heroes.
She joined the team and eventually convinced most of the now-wavering former villains to come over to the "light" side proper. She went on the run with the team, before being "killed" by Scourge. Revived by Techno, she could now become a being of pure electricity, but the damage from the time she spent "dead" left her crippled in her human form, although after much therapy, she fully recovered.
After being stranded on Counter-Earth with most of the Thunderbolts, she chose to stay behind to help the devastated planet as one of the Young Allies. However, Jolt made a brief return to Earth, in order to help both the Thunderbolts and Avengers to stop the out-of-control Moonstone (who had possessed two moonstones at the time and was losing control when she absorbed the Liberator's energies within her).
After her "rebirth", Jolt could change from flesh-and-blood into living electricity. In her energy form, she could fly and fire blasts of electrical force. At first, she could only function in her electric form, her human form was so injured that she could barely walk and talk. She has now fully recovered, and her level of power has increased.