A failed, if fondly remembered toy, and an also ran character remembered only by Grant Morrison

Jemm, Son of Saturn- Rom, Spaceknight

One of the above named was a rather silly looking toy doll marketed by Parker Brothers —in a rare foray outside of the board game category— in 1979. The toy did not survive for very long (maybe it would have if it was more line with dimensions common to "action figures") but inspired a tie-in comic that outlived it by several years. The character can no longer appear in that particular line, as the license to use it expired and was never renewed.

The other is not, notably, the doll by the same name that was spun off into a 1980s cartoon show popular with preteen girls; but it was initially conceived and pitched as new direction for J’onn J’onzz. When J’onzz was said to have been planned for a revival in the Justice League, this character was re-jiggered slightly. His comic didn't make much of dent, and the character was revived by Grant Morrison in his 1990s JLA series.

Commonalities:
Both visit Earth from their native planet, and combat nasty aliens. And both, frankly, are utterly obscure to all but the most dedicated ‘80s fanboy.

Differences:
Rom is a cyborg, and Jemm is an also-ran J’onn J’onzz: both share several attributes nonetheless.

Alternate histories:

JE: Jemm is the first of a race living on the moons of Saturn to volunteer to defend his people from invading Koolars, a subset of the “white martian” race. He and other recruits successfully repel their foes, and Jemm comes to the planet Earth to combat the Martians further. He encounters other superhuman champions and eventually returns to the moons of Saturn to repopulate his now exterminated civilization.

RO: Rom is the heir to throne of Galador, a planet populated by an expatriate race from the planet Zenn-La. When the Dire Wraiths, a subset of the Skrull race, begin to war against Galadorians, Rom goes into hiding and develops his hereditary abilities, and later departs for the planet Earth. After a number of exploits combatting the Wraiths, he returns to Galador, where he is disowned for not taking a side in a civil war therein. Rom has since intervened in galactic affairs from time to time, often alongside distant kinsman the Silver Surfer.