Who's This? He's the Charlotte Brontë character on page 30 of Who's Who's 9th volume.
How you could have heard of him: The limbo joke was made again in Final Crisis Superman Beyond 3D where he was on hand to help Superman save reality. Or you might have heard of these old comics in a Law & Order episode called "Love Eternal" (Season 20, episode 22) where "he was cheerful" became a recurring gag. In any event, he should not be mistaken for the Atlas Comics character also called the Grim Ghost.
Example story: Sensation Comics #4 (1942)
Alas, no. Missed opportunity. But the sometimes-gay, sometimes-grim ghost had to rescue the captured graffiti artist so... priorities.
And so it went, abroad and at home, Nazis Nazis Nazis. No word on whether he and Debbie ever tied the knot, or if that would have been very wrong indeed. As far as the Gay/Grim divide, I'd say Everett was more Gay than Grim. By which I mean cheerful. Because punching Nazis and destroying their stuff while you are completely invulnerable is big fun.
Who else? Between our last entry, the Golden Gladiator, and this one, I might have had a go at Gravedigger or Grimbor, but neither seemed obscure enough. The former can be found is relatively recent war comics, and the latter is a Legion villain, and they're all obscure to at least one segment of the comics-reading population, but not really to another. Still... on the next pass for sure. For now, we're heading into Volume X.