Julie Power was born in Richmond, Virginia to Dr. James Power and Margaret Power. Julie was perhaps the most "normal" of the Power siblings. When she was not in "superhero mode" Julie could be quite shy and quiet. Julie was also a voracious reader, and was frequently seen reading, carrying or quoting books. She frequently stepped into a maternal role with her siblings, caring for Katie and Franklin Richards and attempted to restore peace during conflicts.
In battles, Julie possessed quick reflexes and was a strong fighter. In the Pack's initial conflict with Prince Jakal, Julie was able to single-handedly bring down the Snark's ship. She was also the only member of Power Pack to be directly responsible for the death of another — Pestilence, who fell to her death when Julie struck her with the "Julie Hammer" battle technique.
Having begun super-heroic activities as a child and spent nearly half her life in that role, Julie felt she had missed experiencing a normal life, and decided to retire from being a superheroine and focus more on her private life. During her first meeting with the Loners, she explained that she wanted to become an actress. However, she (as well as the others within the group) agreed to go on the mission offered by Rick Jones to help bring in the Runaways for their own good. After the mission they decided that it was a one time thing and returned to their support group.
Julie however felt the urge to make use of her powers again and dove off a building and soared through the sky. When Julie intruded during a battle between Hollow and Ricochet, she was stabbed through the shoulders by Hollow. Fortunately, her alien metabolism allowed her to heal from the wound quickly. Later, she enjoyed a conversation over burgers with Ricochet and had him take her to Marvel Studios for an audition. Julie failed her audition but caught the eye of an action director who hired her and Ricochet to use their powers as stunt people, which meant she had to lie about having registered under the SHRA. She quit this job quickly though when the director came onto her despite her being underage and completely uninterested and then began trying to be honest with her friends. Julie then helped the team as they fought against Nekra and then Phil Urich when he succumbed to mental illness and stole the Darkhawk amulet from Chris Powell.
Julie, along with many other members of the Loners, joined Avengers Academy. She helped Striker come to terms with his sexuality. Soon after, the Academy came under attack by Hybrid.
She became irritated with Striker when he resumed his egotistical facade and announced his sexuality as a publicity stunt. Later that day, the Academy came into brief conflict with the Runaways, who were searching for the dinosaur named Old Lace. Hostilities ceased soon, and Julie briefly flirted with Karolina Dean before the search for Old Lace continued. After having retrieved Old Lace, Julie and Karolina made plans for a date, and eventually they end up being romantically involved. Later, Julie broke up with Karolina for being inattentive.
After a couple of months, Julie seeped into a bout of major depression due to her break up with Karolina. Her grades took a significant deep which forced her to drop out of Empire State University. Living alone in her apartment in solitude, she became isolated and jealous due to the popularity of her ex and Nico's kiss which was on the front page of the tabloids.
All of a sudden her brother Alex was transported to her apartment via Dragon Man's teleportation ability. He begged her to come with him as he needed help in saving the Future Foundation from dangerous creatures. She agreed but wasn't given enough time to change her attire as Dragon Man brought them back to Asteroid 1984-JA. She freaked out that Alex didn't give her a chance to change her clothes which prompted Bentley-23 to make a snark comment about how the Power siblings were useless. But of course Julie was able to help as the creatures they were being attacked by were photophobic, so she used her light-speed powers to carry each member of the team to safety without any injury from the monsters as her body generated light as she flew.
After they made it back to the ship, Julie went to Alex's quarters to play catch up with what was going on in each other's lives. Julie told him about her coming out as bisexual and how her recent breakup led her down a road of possibly moving back in with their parents or becoming homeless. She conveyed that she had nothing worth going back to at home and asked if she could join the ranks of the FF to help Alex lead the team. He gave her an undeniable yes and the both of them hugged each other with a warm embrace.
At some point, Lightspeed briefly rejoined Power Pack and teamed-up with Devil Dinosaur, Gwenpool, and Moon Girl.
