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He was born on Earth but moves at incredible speeds, has incredible powers and looks very different in comparison to a human. What is he and how did he become like that?

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He was being experimented on , and yes he was a mere human , and also a failed experiment It will all be revealed in the anime if you keep watching

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  • Please include relevant sources/references.
    – W. Are
    May 4 '19 at 2:34
  • ansatsukyoshitsu.fandom.com/wiki/Korosensei May 4 '19 at 6:25