Kara was the Supergirl of Earth-2 and the cousin of Superman. She was also best friends with Helena Wayne, the Earth-2 Robin and Batman's daughter.
Supergirl was one of the heroes who defended Earth from Darkseid's invading forces. She did not get directly involved in the fight; instead, she guarded the nuclear devices that the world's governments had decided to keep to the very last. Superman, Batman and Wonder Woman were killed, but they managed to destroy Darkseid's towers, disabling the Parademons and saving Earth. As Supergirl caught up with Robin, and without any time to grieve their lost mentors and family, their attention was caught by a Boom Tube portal with a lone figure appearing within it. The Earth's last two remaining heroes plunged into the portal.
The portal led them to Earth-0. Karen and Helena established new identities to blend in, but they also sought a way to return home. Karen used her powers quietly, working to buy or "borrow" as much advanced technology as possible. She had since become a public figure and quite wealthy. Also, she assumed the identity of Power Girl to continue her superhero duties.