After the Superboy of Earth-17 mysteriously disappeared, the benign Lex Luthor of that world unwittingly released a trio of Kryptonian criminals – Dru-Zod, Quex-Ul, and Zaora-Ul – from the Phantom Zone. Free of their prison, and without Superboy around to stop them, they wreaked havoc upon the Earth, using their nigh-godlike powers to devastate the surface of the planet and slaughter billions.
As Superboy had been the only superpowered hero on Earth-17, there was no one else on the planet capable of taking on the Kryptonian supervillains. In dire need of a new superhero, Luthor took it upon himself to create one. Using his knowledge of genetics and abiogenesis, he created an artificial humanoid: the Matrix. Modelled after Luthor’s dead lover, Lana Lang, Matrix was completely female in appearance, visually indistinguishable from the woman she had been patterned after. Unlike the late Lana, however, Matrix possessed a host of superhuman abilities: flight; enhanced durability; accelerated healing; and the ability to manipulate light, allowing her to render herself invisible, create holographic disguises, or even generate devastating laser blasts en par with Superboy’s heat vision.
Matrix – christened Supergirl by Luthor – was quickly put to the task of fighting the Phantom Zone escapees. Unfortunately, it became clear that for all her powers, she still was no match for the immensely powerful Kryptonians, and they easily defeated her. Coming to the conclusion that only Superboy could fight these monsters, Luthor set about probing the multiverse in search for one. In time he found Earth-0, which was home to a SuperMAN. Confident that the Superman of Earth-0 would be able to do the job required, Luthor sent Supergirl through a hypertime platform to Earth-0 to recruit him.
After Superman was brought through to Earth-17, Luthor and his allies planned a last, desperate gamble against the Kryptonian criminals. The gamble ultimately failed; Superman was overpowered, Supergirl was grievously injured, and the resistance fighters were all slaughtered. To add insult to injury, the Kryptonians decided to finish the remaining humans off; using their immense physical strength, they drilled holes through to the Earth’s core, unleashing a cataclysm which killed the planet and everything on it.
With the entire world dead, Zod and the Uls decided to use Luthor’s hypertime platform to travel to Earth-0, where they would continue their genocidal massacre. Without any further choice, Superman used the green kryptonite of that universe – which was harmless to him – to execute the criminals, finally bringing their reign of terror to an end. Taking Matrix – who had been burned down to a barely human crisp – with him, he left Earth-17 for his home universe.
Once back on Earth-0, Superman placed Matrix in the care of his parents. Adopting her as a surrogate daughter, they decided to name her Mae Kent and began nursing her back to health. After several months passed, Matrix healed from her wounds, but the damage done had left its mark on her; her hair, which had been a fiery red, had been permanently altered to a light blond.
A number of years passed, and in May of 1959, Mae Kent made her public debut as the Earth-0 Supergirl, adopting an updated version of the costume she had lost on Earth-17.