She was a superhero operating on Terra, a world almost identical to Earth but with subtle differences. In a parallel origin to Superman's she was a survivor of an underground civilization which was destroyed. Her father Jaal-Kor, wishing to spare his daughters life, sent her to the surface world in a drilling device, where she gained incredible and became a superhero on Terra. Althought she was the equivalent of Supergirl, bearing many similarities to her, she was the primer superheroe of the world, same as the role Superman has on Earth, including having a secret identity, Leah Lindy and a job as a cub reporter in the Macropolis' Daily Planet.
Her base of operations was the orbiting Fortress of Marvels, and her home city on Terra was Macropolis. Her cousin,Marvel Man, was her partner and secret weapon, in similar fashion to the Silver age Supergirl role.
When Supergirl learned of the existence of Terra, she would go there with Superman's permission to ask Marvel Maid if she can take her place temporarily and see if she can prove to Superman that she can operate as a super hero and not make any mistakes. Marvel Maid agreed, however the subtle differences between Terra and Earth would trip up Supergirl who would in the end still manage to correct her mistakes and save the day. Marvel Man would come to her rescue, when her x-ray vision set an ice berg ablaze, and he would secretly try to assist her while she in turn trained him how to use his powers. He would almost be killed when Supergirl unknowingly turned coal into a diamond to distract a family from spotting them not knowing that it was the one weakness of Marvel Man.
After working together on numerous cases, Supergirl would later ask Might Maid to petition Superman on Earth to allow her to reveal her identity to her adopted planet given her performance on Terra. Marvel Maid agreed, however the subtle differences between Terra and Earth would trip up Supergirl who would in the end still manage to correct her mistakes and save the day.