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 attempting 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 superheroic 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 Excelsior, 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 the decided that it was a one time thing and return to there 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, 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 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.
Self-Propelled Flight: Julie can fly at supersonic speeds while creating a stream of refracted light. After regaining her powers from her sister Katie, Julie learned how to hover in the air without accelerating.
Speed: Julie's maximum speed is at least 800mph, faster than the speed of sound. She has been shown creating sonic booms when flying.
Rainbow Trail: Julie also emitted a distinct rainbow colored trail while flying. She eventually learned to manipulate this trail so that she could stand on it, use it to cushion the falls of others, and even as a hammock.
Teleportation: The second manifestation of Julie's Acceleration powers, which manifested after Julie had a strong desire to be in two places at once. This power allowed her to traverse great distances, seemingly without effort. She teleports in a flash of pink light.
Limited Invulnerability: Lightspeed has survived direct energy blasts from Ultron, which previously disintegrated Victor's mother, meaning that Julie may have been able to draw on the powers of her siblings even though they were not nearby. Previous incidents have demonstrated a sharing and linking of these powers when the siblings are in proximity to each other.
Density Control: (formerly) After switching powers with her siblings, Julie gained her brother Jack's density control powers. This allowed her to manifest the powers that were originally used by Jack, such as the ability to become a super-dense cloud that allowed her to fly and avoid attacks. She could also shrink down to 8 inches in height which made her nearly invulnerable to assault. In addition to the previously established powers, Julie refined the Density Control powers by learning to create density shields which allowed her to protect herself and others from attack. Julie could also increase her size while retaining the same mass, but this power exhausted her the longer she used it. Julie and her siblings eventually had their powers switched again, and she regained her acceleration powers.
During Julie's time with the Loners there were hints that she might be unsure of her sexual identity. This was not resolved in the series itself, but in Avengers Academy series, it was revealed that she considers herself to be bisexual.
Julie was the only Power family member who had a birthday happen within the comic series, aging from 10 to 11 years old. Later she is nebulously identified as being an "ex-teenager", but later still Julie identifies herself as being 17 years old. Recently Julie has been identified as being about the same age as the members of Avengers Academy.
Julie's favorite book is The Hunting of the Snark by Lewis Carroll.