Metallo Superman the Animated Series
"There's the reality! The metal behind the man! It's all I am now! It's who I am... Metallo."
John Corben was a mercenary and hitman who became the cyborg Metallo.
John Corben was originally a top-tier mercenary. At one time, he led a team of mercenaries to steal the Lexo-Skel Suit 5000 from LexCorp, a theft staged by Lex Luthor for the benefit of the Regent of Kaznia. When Lois Lane tracked the thieves to their ship in Metropolis Harbor, Corben caught her and prepared to kill her, but Superman intervened. To give himself a fighting chance, Corben mounted the Lexo-Skel himself and engaged Superman in a fight that raged across Metropolis, ending when Superman tore the suit apart.
Although Luthor represented to the Regent's lawyer that he had fulfilled his part of the bargain by letting "the Regent's men" take possession of the suit, Luthor subsequently acted as though Corben had been working for him.
Corben was sent to Stryker's Island Prison, where he refused to give details on his employers. For this reason, his cell featured ample creature comforts and gourmet meals, supplied by an apparently grateful Luthor. A few months after his imprisonment, he was shocked to be told he was suffering from a rare and terminal viral disease found only in a small island in the South Seas. While looking at his prison record, Clark Kent was perplexed how John Corben could have contracted this disease, not being native to the tiny island where the only cases are known. Lois Lane chalked it up to the fact that Corben was an international mercenary with outstanding warrants in several nations, thus it made better sense that with such globe-trotting Corben would be more at risk for exposure to rare diseases.
Luthor helped Corben escape, and in exchange for his discretion, offered him a way to avoid death: LexCorp scientists would transfer his mind into an indestructible android body made of metallo, possessing strength equal to Superman's and powered by a Kryptonite heart. Feeling he had nothing to lose, Corben agreed.
Corben enjoying his newfound strength.
At first, Corben was thrilled with his new abilities, which allowed him to fight Superman nearly to the death. However, when he realized that he could no longer smell, eat or feel, he became disenchanted and abandoned the fight. Corben sought out LexCorp scientists, demanding that they make the "adjustments" they talked about when he awoke in his new body. However, the scientists confessed that they really meant Corben himself had to adjust to his transformation, which was irreversible. Upon hearing this news, Corben went insane, ripping away part of his artificial skin and officially adopting the identity "Metallo".
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