Supergirl began as an idea, originally as a way to extend the legacy of 'The World's Greatest Hero', namely Superman, the Man of Steel. Unlike Superman, it's uncertain who actually created her, as this has been attributed to the persons responsible for her first 'official' story in Action Comics #252 (writer Otto Binder, editor Mort Weisinger, and artist Al Plastino). It seemed that in the 50's, anything connected with the character of Superman couldn't possibly go wrong, so the creative forces behind this character started to add new concepts and ideas to his mythos, in order to expand the possibilities for future exploits. Stories of Superman as a boy (known as 'Superboy' stories during this time) had begun in 1949 with a good deal of success, so it was probably only natural that National Comics (DC Comics' trade name in the 50's) began to look at a super-powered female version. Another possibility for the idea may have stemmed from the Captain Marvel mythos, whose Golden Age family included Mary Marvel, 'The World's Mightiest Girl'. There were also a few Superman stories where his paramour Lois Lane would acquire super-powers. Yet there was still some trepidation into adding a super-female, as it would require some major tinkering with Superman's existing mythos (particularly the idea that he was 'The Last Son of Krypton') in order to make the new concept fit.