There are some characters that are shrouded with mystery in the Champions Universe. For me, Mechanon and the Monster are the first to come into mind.
So what are the origins of such characters in your campaigns?
I'll start with my origin of the Monster.
One of the former characters a player used before we had one of our campaign shifts was a character called the Boogeyman.
He was basically like Spiderman mixed with Batman and with a hint of more creepiness and brooding, but he wasn't really "iron age."
So here's what I did.
I have taken cues from the Mystic World to create my Antimaterial Plane which = DC's Anti-Matter universe or Marvel's Counter Earth.
There's the Sephirothic Tree Of Life, and then there's the Qliphothic Tree Of Death.
The Antimaterial Plane is the only Qliphothic plane that can be reached through non-magical means. This is because, just like Assiah, it doesn't run on magic, or anti-magic or whatever.
And like pretty much any anti-matter dimension, it contains opposites.
The Boogeyman belonged to a super-group that preceded our current one, called the Fantastic Uniters, and they disappeared after an adventure battling Bowser, the King of the Koopas in a cybernetic dimension(but that is a different story).
The Boogeyman is all about preparation, and quick wittedness, but like Batman, he's still human for the most part.
But on the Antimaterial plane, his opposite is the Monster. The Boogeyman preys upon criminals using his wits and technology and skill where the Monster preys upon the weak and innocent on Anti-Earth using brute strength and pure evil.
The Monster belonged to an up and coming super-team called the Phantom Syndicaters, and they mysteriously disappeared when battling a supernatural menace.
The Monster and his allies all appeared on our Earth, and are looking for each other. His mind is difficult to penetrate, and is downright harmful because he is from the Qliphothic planes, and is far from human.
I'll save Mechanon for later.