In May 1959, Superman was no longer unique amongst his kind. A fellow Kryptonian joined him in the fight to protect the Earth, only this individual was his cousin... and a female. Kara Zor-El, the last survivor (well at least that's what she thought) of the floating remnant city of Argo from long extinct Krypton, was sent to cousin Kal-El's new home world to assist him in his crusade to protect the innocent using an array of superhuman abilities. She was quickly dispatched into an orphanage until she was adopted and raised to adulthood by the Danvers family, unaware of her secret identity.
In January 1976, the Justice Society of America was revived in a semi-monthly format, and along the way they decided to introduce an Earth-Two version of the Maid of Might. This Kara, last name Zor-L, was the last survivor of Kandor who was several decades younger than her cousin. How could this be, when Krypton was destroyed in the early 20th century of this universe? This would be a mystery that would be closely guarded until years later during a solo run of Power Girl's in Showcase. In the interim, she developed a distinctly different persona than that displayed by her Earth-One counterpart. While Supergirl was alwasy eager to impress and emulate her famous cousin, Power Girl slowly began to resent Kal-L for his seeming overbearing attitude. Perhaps this was due to the fact that the only other Kryptonians that he had met in the flesh had been U-Ban, Mala and Kizo who were three exiled evil scientists. Seeing Kara's independent disposition no doubt caused him some pause, yet regardless he brought her into his home to tutor the socially ackward gal alongside his wife Lois Lane Kent, who became like a mother to her. Supergirl's alias on Earth was Linda Lee Danvers, while Power Girl's was Karen Starr.
Early in their career both Karas met their world's respective Justice teams, and even aided them on their first official team-up with them in defeating to mental mutants... Despero and Brain Wave! While Supergirl preferred to remain a solo heroine, perhaps in part to help her define herself from Kal-El, Power Girl readily jumped in the fray and immediately tagged along with the JSA until she was soon thereafter accepted as Kal-L's replacement at his request! Supergirl herself was a vagabond of a hero, traveling to various locations including Florida where she was a guidance counselor and New York City were she was a soap opera star before her final nesting place of Chicago where she went back to college. Power Girl herself gained instant education thanks in no small part to a memory teacher that Wonder Woman allowed her to use on Paradise Island, which provided Kara with a solid computer programming foundation. She then joined a tech firm in Gotham City, working alongside her her buddy the Huntress, before moving to Metropolis to start her own company. She also on occasion operated in Los Angeles.
In addition, both Girls of Might joined two teenage teams, the Legion of Super-Heroes and Infinity Inc. after having been partly recruited by a couple of brainy guys, Brainiac Five and Brainwave Jr. However they only stayed attached to them on an honorary basis... after all these teams didn't operate in their locale... Infinity Inc. was on the west coast directly opposite Power Girl's home base of Metropolis while the Legion operated on the east coast... but in the 30th century!
While both blonde bombshells were deep down found of their cousins, there were times when they came to blows particularly when both Men of Steel temporarily lost their minds alongside their fellow JLA and JSA members. Each held her own. Years later, Supergirl learned that her parents Zor-El and Allura and survived in a type of Phantom Zone until she released them and they began to live in the bottled city of Kandor. Also years later, Power Girl's secret origin was revealed, as she had been sent from her planet at the same time as Kal-L although her vessel came equipped with two distinct components his did not possess: a suspended animation component and a virtual reality device. The former kept her from aging beyond twenty years while the latter allowed her to experience life on "Krypton", albeit a virtual reality construct of said planet. This included interacting with her fellow Kandorians (she had moved from Argo City as an infant) including Zor-L and Allura until her Symbioship vessel eventually brought her to Earth.
The Crisis on Infinite Earths proved to be a devastating experience for both women, as Supergirl perished in battle against the Anti-Monitor while Power Girl's history was wiped out as her planet merged with Earth-One. Still, their legends prevailed and each would return in her own distinct way to once more battle evil. And eventually meet, which they never had previously.