Metallo is a prominent enemy of Superman and a supervillain from DC comics - he is one of the Man of Steel's more dangerous opponents due to the fact he has transformed himself into a cyborg that is powered by green kryponite - a substance that is deadly to Superman in large doses and severely weakens him whenever he becomes exposed to it.
Metallo started his life as a simple criminal but became a much more powerful villain after having his body upgraded - due to his status as one of Superman's main enemies Metallo has appeared in many comics, TV series and videogames based around the franchise: some of the more famous examples are his appearances in the Superman animated series, the Justice League and Smallvile - he also appeared in The Batman series.
A professional conman, John Corben was fatally injured in a car crash. Professor Emmet Vale, a cybernetics specialist, found the dying Corben and transplanted his brain into a cyborg body powered by a chunk of Kryptonite. When Corben, renamed Metallo, awakened, Vale instructed him to kill Superman. Metallo turned on Vale and killed him, but nevertheless went to battle against Superman. After his defeat, he was captured by Lex Luthor, who tore the Kryptonite chunk out of his chest, seemingly killing him.