Power Girl is the Earth-Two counterpart of the Kryptonian Supergirl and first cousin to Kal-L, the Superman of Earth-Two. Possessing her cousin's superhuman strength and ability to fly, she took his place in the Justice Society of America after he entered into semi-retirement to focus on his personal life. Kara-L has since been recognized as an important member of the JSA.
After the events of the Crisis on Infinite Earths and the merging of Earth-Two into New Earth, Kara's background was completely altered. The reality wave that eliminated the Krypton of the Earth-Two dimension changed her into the descendant of the Atlantean sorcerer Arion, having been frozen in suspended animation for millennia until the present day. This was later revealed to be a deception, and Kara's true Earth-Two heritage was revealed to her during the "Infinite Crisis".

After discovering Krypton was about to explode, Kara's father Zor-L launched her as an infant to Earth in a ship at the same time that Kal-L's ship was launched. However, Kara's ship took a much longer journey, resulting in her arriving on Earth decades after her cousin. According to Kara, the trip took 60 years to finish.
Unlike Kal-L's ship which was designed for fast travel, Kara's "Symbioship" was designed to hold Kara in stasis during her development through the prolonged voyage. In addition, the ship's artificial intelligence had the life experiences and education of a Kryptonian in the form of a virtual reality program wherein she could interact with life-like copies of her parents and fellow Kryptonians. When she landed on Earth, Kara was the age of an adult woman in her early twenties.

Power Girl's existence was not revealed to the general public until years after her arrival on Earth-Two. Her cousin Clark and his wife Lois provided her a family environment to assist her transition toward real life relationships. She came to regard her cousin and his wife as her "uncle" and "aunt" for raising her on Earth-Two despite being cousins by blood and marriage. Lane-Kent stated that they regarded her as their daughter.

In her first recorded case, Kara assisted Justice Society members Flash and Wildcat in containing an artificially induced volcano that had erupted in Peking, China. She then joined them and other young heroes Robin and Star-Spangled Kid to form a Super Squad, along with active members of the Justice Society, to defeat Brain Wave and Per Degaton's scheme for world domination. Later, she was granted full membership when she took over Superman's vacated spot on the team roster.

Having been raised by the Symbioship with artificial Kryptonian life experiences, Power Girl had trouble fitting into life on Earth, but with the help of reporter Andrew Vinson, she eventually adopted the secret identity of computer programmer Karen Starr. She developed her knowledge in this field from treatment under Wonder Woman's purple ray on Paradise Island. She never wore a mask or other disguise between her two identities and, at some point, her identity would be revealed to the public. During this time, Kara developed a close friendship with Helena Wayne, the daughter of the Earth-Two Batman and Catwoman.

After the Justice Society disbanded, Power Girl joined the Justice League. Later, while a member of Justice League Europe, she was badly injured by a mystical being. Superman assisted in her medical treatment, using his heat-vision to perform surgery on her otherwise-invulnerable tissues. Although she recovered, she was significantly weaker, lost all of her vision powers, and temporarily lost her ability to fly.

During Zero Hour, she was mysteriously impregnated. She gave birth to a son, Equinox, who aged rapidly through artificial means. He later disappeared, never to be heard from again.
Power Girl was recruited to be one of Oracle's first field agents. Their short-lived partnership ended in failure after a disastrous mission that resulted in a large loss of life. Power Girl believed that bad orders from Oracle were to blame for the tragedy and disliked Oracle intensely as a result (although she has worked with her again on a few occasions when needed).

During the events of Infinite Crisis, Power Girl was shocked to learn from Arion that her Atlantean origin was a lie. The story was crafted for her own protection, and was created by her foster mother, Lois Lane.
The Psycho-Pirate showed Kara multiple origins in an effort to drive her insane, but eventually revealed that her Kryptonian origin was her true origin. Power Girl was not only a survivor of Krypton, she was also a survivor of the entire Earth-Two universe. It is still unclear why she survived the Crisis retaining her pre-Crisis origin, when other Earth-Two figures, such as Jay Garrick and Alan Scott, did not.
Kal-L himself returned to the mainstream universe after breaking down the walls of the paradise dimension in which he, Lois Lane-Kent, Alexander Luthor, Jr., and Superboy-Prime had been living since the end of Crisis on Infinite Earths. Appalled by the rapidly-deteriorating state of affairs on the contemporary Earth, their goal was to replace the post-Crisis planet with a recreated Earth-Two. Kal-L's first order of business was to track down Power Girl and explain the events of the original Crisis to her. He also reiterated her pre-Crisis history as his cousin. A touch from the ailing Lois Lane-Kent inexplicably restored Power Girl's memories of Earth-Two.
Soon after this revelation, she was confronted by Superboy-Prime, who rendered her unconscious. In her sleep, she was attached to a "tuning fork," a device controlled by Alex Luthor, who intended to use the device to bring back the multiple Earths. Alexander Luthor, Jr. and Psycho Pirate coerced Black Adam (who was also attached to the machine) into saying "SHAZAM!," and used the now-raw magical energy to power the tower. After the recreation of a duplicate Earth-Two, everyone associated with that Earth was subsequently transported to it. However, Power Girl would remain on the core Earth reality because of her proximity to the tower.
After arriving on the recreated Earth-Two, Lois Kent collapsed and died. This led to a violent confrontation between the two Supermen, at the end of which, Kal-L came to the realization that the recreated Earth-Two had not been a "perfect world" because "a perfect earth doesn't need a Superman."
Kara was freed by Wonder Girl and Kon-El and joined them in fighting Superboy-Prime and Alexander Luthor, Jr.. During the savage battle on Mogo between Superboy-Prime and the two Supermen, Kal-L was killed. Power Girl arrived on Mogo with the Green Lantern Corps just in time to bid a tearful farewell to her dying cousin.

Kara's recognized history as an Earth-Two being was re-established as her natural state. How she continued as an Earth-Two dimensional being despite everyone else from the same original Earth-Two dimension being destroyed and reformatted to current Earth natives has NOT been specifically clarified. She has been retconned into being powered by yellow sun radiation, as was her cousin, Kal-L, in Infinite Crisis, where flying through a red sun depowered him.

To further complicate the matter of her basis and identity, Kara is now to have been raised as the cousin of the current Superman and living with the current Kent Family and inspired by her cousin to join the Justice Society as done by Kal-L in the Earth-Two dimension where her cousin Kal-L was training her and announced her superhero appearance. Currently Kara remembers living through both histories.

Power Girl was shown to have been working under the guise of Nightwing in the dimensionally separated Kryptonian city of New Kandor. Kara-L was working alongside her dimensional counterpart, Supergirl, who had adopted the alias of Flamebird while on New Kandor. When they left New Kandor, Kara-L and Supergirl had revealed that the supporter of the cult of the Saturn Queen was not the Primary Earth Superman but Ultraman operating under the guise of Superman's identity. After exposing Ultraman, Supergirl decided to leave New Kandor. Power Girl wanted to stay and overthrow them completely. Supergirl dragged Power Girl out of New Kandor, which led to a strain between the two. Power Girl went on to help re-form the current roster of the Justice Society of America, where she was elected Chairperson of the team. Currently she is helping the Earth-22 Superman adjust to life on New Earth and coming to grips with the death of her cousin Kal-L.

When confronting the super-being known as Gog, Power Girl was sent to the parallel Earth known as Earth-2 as part of Gog's understanding of "sending her home." Thinking she had somehow returned to her home reality of Earth-Two, she sought to adapt to life there with the counterparts to her long dead (and historically erased) friends. But she noticed several major differences to her own reality in the form of Helena Wayne, who was going to murder the Earth-2 Joker. Kara took the brunt of the Joker's electrical attack, preventing Helena from actually doing the deed and saving Helena from making a terrible mistake. However, Helena immediately sided with the long missing Earth-2 Power Girl who brutalized Kara Zor-L. Escaping, Power Girl defeated her crazed doppelganger from Earth-2 before being felled by a mental attack from Brainwave Jr., who revealed to the JSI that the two Power Girls were in fact the same person up to a point. The coming of the JSA with Starman, eased the situation, and the JSA took their leave of Earth-2 with their Power Girl in tow. Starman revealed to Stargirl that the JSI would play a significant role in Power Girl's future.

Power Girl was trying to avoid fighting her unbalanced Earth-2 counterpart and her associates of the Justice Society Infinity as she sought to return to her adopted reality and friends. Once back in the New Earth reality, Power Girl resumed her position as leader of the JSA. She also began devoting more time to her old R&D firm, Starrware Labs. She has since resigned as leader of the full JSA team and has formed a subset team known as the JSA All Stars which she leads composed of the younger members of the full JSA team including Magog.

Kara was recently fighting against her resurrected "aunt" Black Lantern Lois Lane-Kent of Pre Crisis Earth-Two, who is seeking to kill the New Earth Martha Kent and Lois Lane-Kent and uncle Kal-L. She was instrumental in completely destroying Kal-L's corpse so it could not ever again be used as a weapon in the future as it was by Necron. Though that destruction also meant Kal-L's body was forever destroyed and not returned to its grave on New Earth.