James Moriarty

James Moriarty was the archenemy of Sherlock Holmes.

As a real person
In order to investigate Maupertuis, Moriarty used the alias O'Connor and donned a ginger wig as part of his disguise. He also wrote a paper on the mathematics of folding higher dimensions. (NA: All-Consuming Fire) Moriarty died in 1893. (IW: The Found World)

As a fictional character
Contradicting other accounts, Moriarty also seemed to exist in the Land of Fiction. (DWM: Character Assassin)