Kara Zor-El was the daughter of Kryptonian scientist Zor-El and Allura In-Ze. She was born on Argo City, a part of Krypton that had been flung free when the planet exploded. However, the city's land slowly transformed into the poisonous radioactive green isotope called Kryptonite. Zor-El used his scientific expertise to cover the planetoid with lead sheeting to protect the people.
Later, a meteor shower punched holes in the city's protective bubble and the lead sheeting. Poeple began dying. To save Kara, Zor-El constructed a rocket to send her to Earth, where, he had discovered, his brother's son, Kal-El, had grown up to become Superman.
When Kara's rocket landed on Earth, Superman found her. He created a secret identity for her, Linda Lee and placed her in Midvale Orphanage, but allowed her to aid him secretly as Supergirl. After a few years, she convinced him to allow her to operate publicly. She was also adopted by kind foster parents, Fred and Edna Danvers. Later, her real parents were found to have survived, by entering another dimension called the 'Survival Zone', similar to the Phantom Zone, where they had been trapped. They were freed and went to live in the Bottle City of Kandor.