Marvel Man is a character than existed before the Crisis in Infinite Earths. He was a Silver Age character from a world pretty much similar to Earth know as Terra, a planet that is light years far away. He was pretty much identical to Superman in image and origins but with a twist: he was the sidekick to his cousin, Marvel Maid, the premier superhero of Terra.
Coming from an underground society on the planet Terra,this world was soon doomed when the caverns that housed their homes crumbled. His cousin's father Jaal-Kor, wishing to spare his daughters life, sent her to the surface world in a drilling device. Marvel Man would share a similar fate being sent to the surface in another drill device but the device malfunctioned trapping him in suspended animation until he reached adulthood and was freed. Meanwhile, his cousin would become the hero Marvel Maid.
Finding her long lost cousin, Marvel Maid decided she would someday need him as an ace in the hole against a super-villain and seeing that he needs a lot of time to learn how his new found powers work, she set him up with a civilian identity of his own: Ken Clark, a prisoner, who would secretly break out of jail when needed as Marvel Man. Marvel Man would operate in secret much like the Silver Age Supergirl of Earth would.
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.