The Man With The Kryptonite Heart!
He's been menacing Superman on and off since 1959....but what do we really know about him?
Well, let's start with his beginnings!
A Real Man of Steel
This accident happened to Roger C. Corben, the brother of the original Metallo, who had joined S.K.U.L.L. to get revenge on Superman, and S.T.A.R. Labs saved Corben's life by making him into a cyborg as well, and he was also powered by Kryptonite.
In Superman #316 and #317 (October-November, 1977), Metallo did just that (in stories by Pasko and Swan, and stunning covers by Jose Luis Garcia Lopez and Neal Adams) and planned to take Superman's heart after the Man of Steel died from his Kryptonite trap (it didn't work out).
Metallo ended his pre-Crisis career with an appearance facing Superman in Superman #418 (April, 1986), being one of many Superman foes who didn't appear within the pages of the Crisis on Infinite Earths, which caused Superman's history to be changed (which was detailed a little in the Man of Steel mini-series, and led to...).
New Earth, New Man
This Metallo was more of a Terminator type robot, and kept coming back. John Corben was a con man who was injured in a car crash, and restored by Professor Emmet Vale (as the cyborg Metallo).
In thanks, John killed Prof. Vale, and then went to attack Superman....who was saved by Lex Luthor (because Luthor felt only he had the right to kill Superman!).
Mostly, Metallo was used by another villain...and Superman defeated him by taking his head off (his brain was on a separate system, so the villain survived, just wasn't threatening as a head!).
This put Metallo into conflict with Steel (the Superman substitute that became a hero on his own) in Steel #21, as well as Superboy and Robin in Superboy/Robin:World's Finest Three #1 and 2 (there, working with Poison Ivy).
Metallo had started to be a "big man", but compared to all of Superman's other foes, he was never quite the biggest!
Yet, not unlike Superman, Metallo also came back, time and again....
Seems Metallo will always return to plague Superman....he's as persistent as a heart attack!