Originally Posted by
Doctor Bifrost
In many ways I'd really have to go with the Martian Manhunter. Originally he was a Martian, unwillingly teleported to Earth, who had no way to get back home. So he set himself up as a superhero on Earth, with a secret identity as a seemingly human police detective. And had a touch of melancholy because he yearned for home.
Now, that's fine in self-contained stories. But once he joined the Justice League of America, and was friends with Superman and Green Lantern (who can travel through space), it just didn't hold together too well.
So they kept changing his origin. The White Martians had conquered the Green Martians in a war, and he be killed if he went back. He was teleported through time as well as space, and in this new time the Martians had all left Mars. Or died off. His evil twin bother destroyed the Martian race with a telepathic fire plague, just as J'onn was being pulled away. Or... we could go on at some length, couldn't we? And once he accumulated so many conflicting origins, writers came along and thought, "Hey, I'll just think of a new one!" (In this he somewhat resembles poor Donn Troy.)
And his powers kept shifting, at least in power level. It's one thing to say a humanoid alien can shapeshift into various human forms of roughly the same size and shape. It's another to decide he can shapeshift into anything, from a cat to a huge Lovecraftian monster. (I'm not saying "it's impossible," I'm saying it changes the tone of the character a great deal.) It's one thing to say he has telepathy, so he doesn't have a problem not knowing Earth languages. It's another to use him as the telepathic core of the whole JLA, no matter how far they're separated, and - have I seen this? - having him telepathically influence the whole planet. I mean, power inflation is a thing, but he's got so many powers to start with that it gets a little difficult to describe the core of the character.
Oh, yeah, and it turns out that his true form is this weird Gumby-like creature, and he only took on the form we were familiar with since 1955 to put humans at ease. (Which raises the question of why he went for green and hairless, when he could have just put people at ease by looking like a human being.)
In the past 15 or 20 years he's been given so many backstories that I have little idea, when I see him in the JLA, exactly who he is supposed to be.