Well, after years of stories which teased a female counterpart to Superman, and testing the concept in the previous year, DC finally pulled the trigger in Action Comics #252 (March 1959) with the introduction of Supergirl into the DC continuity. Supergirl is Kara, daughter of Zor-El and cousin of Kal-El (Superman). Her father’s city survived the explosion of the planet Krypton and tumbled through space in a protective bubble with sheets of lead to block their homes from kryptonite radiation. Zor-El eventually settled down, took a wife and had a daughter, Kara.
Unfortunately, the metal until the radiation of its soil broke through the sheet metal that had been deployed to shield the inhabitants. Zor-El sent his daughter to Earth where she could be with her cousin, the only other known survivor of Krypton.
Adopting her as his own child would complicate matters for Clark Kent, confirmed bachelor, so Superman sets her up with a secret identity as an orphan. She chooses the name Linda Lee… DC continues to have fun with those initials L.L. Supergirl agrees to keep her presence in the world a secret for now, while she learns how to use her powers.
Supergirl would continue as an ongoing backup story in Action Comics for years, leaving Tommy Tomorrow just a single spot in World’s Finest.
Known surviving Kryptonians: 1,000,007.