Jerome "Jerry" Siegel, who also used pseudonyms including Joe Carter, Jerry Ess, and Herbert S. Fine, was the American co-creator of Superman (along with Joe Shuster), the first of the great comic book superheroes and one of the most recognizable icons of the 20th century. He and Shuster were inducted into the comic book industry's Will Eisner Comic Book Hall of Fame in 1992 and the Jack Kirby Hall of Fame in 1993.
Supergirl finds an almost identical copy of Earth, a planet called Terra far off in the universe. She decides this would be the perfect opportunity to test her abilities as a superhero while maintaining her secret status on Earth. Superman agrees, and so Supergirl excitedly leaves the Earth to visit this world where she meets her doppelgänger, Marvel Maid.
*I am reading one Supergirl aka Kara Zor-El (Earth-One) story every day, starting with her introduction in May 1959 in Action Comics #252, until her final appearance in October 1985, in Crisis on Infinite Earths #7.