Bio: John Corben was a hit man involved in Lex Luthor's plot to hand in the LEXO-SKEL SUIT 5000 to Kasnian terrorists. Corben fought Superman with the Suit but was eventually defeated and arrested.
While in jail, Corben contracted a rare and fatal disease. Subsequently, Luthor offered him a deal: he would have his mind transplanted into a robotic body powered by a kryptonite cell, and in exchange, he would get rid of Superman for him. The cyborg was made of a virtually indestructible alloy called metallo. At first, Corben rejoiced with his new body and abilities. He confronted Superman and realized that he was as strong as the Man of Steel, and if he used the kryptonite that powered his body, he could even kill Superman, if given enough time. However, once he realize he had lost any sense of feeling, he went mad and took on the name of the metal he’s now made of... Metallo.
During a confrontation in Lex's yacht, Superman revealed to Corben that it was Luthor who had arranged for him to get infected. At this point, Corben turned on Luthor, but when the yacht blown up, Metallo was thrown off to the see and his heavy metal body sank.
Metallo walked across the ocean floor for months. The long time exposure to the pressure of the ocean’s depth gave Metallo amnesia, and when he finally walked up the shore of a small island, he has not memory of who he or Superman is. Coincidentally, Superman was helping to evacuate that island was as a result of an imminent volcanic eruption. In time, Metallo recovered his memory, and fought Superman. However, in the course of the struggle, Metallo got himself caught and trapped in solidified lava.
Metallo was eventually salvaged and upgraded by Intergang. At this point, Metallo was more determined than ever to destroy Superman, and on one battle, he almost succeeded, if not for John Henry Irons, AKA Steel III.
For further character development, refer to Metallo's bio in the Justice League Unlimited's character guide. 
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