What obscure character, who never really made it big as either a hero or a villain, had some of the most interesting development for you?
Aside from Sleepwalker (which should be obvious coming from me), my vote would be for Anthony Davis, also known as the Ringer, and later as Strikeback.
What do I like about his development?
Well, at first he was a petty, jealous, sniveling crook, who got into crime to rob the Richmonds, until Nighthawk clobbered him.
Now saddled with a nasty inferiority complex, he obtained his new battlesuit, and was walloped twice in one day, first by the Beetle, and then by Spider-Man, who both made him feel like a worthless loser.
His confidence shattered, he sinks into petty crime in the Midwest, becoming nothing more than a two-bit hood who tries desperately to prove that he isn't as pathetic as everyone says, until the Scourge finally puts him out of his misery.
Oddly enough, Davis was the only one to survive the massacre, and gets a new lease on life from AIM, who rebuild him as Strikeback. Davis's near-death experience seems to have toughened him up, as he successfully completes several missions for AIM before striking out on his own in search of his wife Leila, determined to end her life of crime.
The fumbling, whiny loser of the past is gone, and the new Anthony Davis is a ruthless, confident ass-kicking machine. He takes out no less than four Spider-villains in one fight; Swarm, Boomerang, and the Vulture all fall before him, although his systems are critically damaged by Stegron. Even so, he finally defeats the dinosaur man before escaping with Leila.
Badly damaged, Davis's systems slowly break down, but his final hours are peaceful enough with Leila at his side, and he dies at peace with himself.
When you think about it, it's actually a pretty interesting character arc-Davis starts out as a snivelling, selfish, two-bit loser who deserves everything bad that happens to him, but later redeems himself by turning Leila away from her criminal ways.
Oddly enough, Leila herself would be redeemed as well, making the ultimate sacrifice in fighting Graviton. Yes, she failed, but she died honorably, fighting a true villain.
The Davises never really became good guys, but they did undergo tremendous personal changes and managed to redeem themselves in their own ways at the end.
Those are some of my picks for Marvel's best-developed obscure characters.
What are yours?