Download Superman #123
She started as a wish.
In Superman #123 (August 1958)1, Jimmy Olsen is given a magic Native American totem that grants its owner three wishes once every hundred years if rubbed under a full moon. (This sort of thing happened to Jimmy on a daily basis back then.) Naturally, Jimmy’s first wish is for a female companion for Superman. And lo, the beautiful blonde Super-Girl appears (note the hyphen). She winds up being more of a hindrance than a help to Superman, however, and after sacrificing herself to save Superman from a giant chunk of kryptonite, she begs Jimmy to wish her out of existence, which he tearfully does.
This wasn’t the first time a female character with Superman’s powers showed up in a one-off story. That distinction probably goes to Action Comics #60 (May 1943), in which Lois Lane dreams she’s Superwoman after a blood transfusion from Superman gives her superpowers. But Super-Girl was created specifically to test reader response to the idea of a young female sidekick for Superman. It must have gone over well, because less than a year later, the first “real” Supergirl appeared.
*courtesy of It's a bird! It's a plane! It's Supergirl!