Marvel Man was largely unknown when he joined the Defenders for a Day. During his debut in Captain America #217, the character had introduced himself as Marvel Boydifferentiating himself from an earlier hero with that same nameonly to become Marvel Man in #218.
The original Marvel Boy had appeared in comic books published in the 1950s by Atlas Comics (the immediate predecessor to Marvel Comics). The character's origin story tied to recent history. In 1934, to escape the threat of Hitler, scientist Matthew Grayson fled Earth in a spaceship, taking his infant son with him. They arrived on Uranus, where young Bob Grayson developed intuitive telepathic abilities, like the planet's peaceful inhabitants. As a young adult, Bob returned to Earth to stop crime as Marvel Boy, wearing Uranian wristbands that could generate a blinding light.
The character made a jarring return in Fantastic Four #164-165. Now calling himself Crusader, the former hero went on a rampage and then vanished, leaving behind his wristbands. From there, Anthony Stark's crew developed the wristbands worn by the new Marvel Boy/Man in Captain America #217-218.
Following the events in Defenders #62-65, Marvel Man guest-starred in The Incredible Hulk #233, with his name highlighted on the cover. Marvel Man could use his wristbands to fly and manipulate energy in various ways. His confidence plummeted, however, when a boy told him that Marvel Man was a dopey name. In #234, the hero changed his name to Quasar, distinguishing himself from other recognizable Marvel characters, including Captain Marvel and Ms. Marvel.