* Harry Potter: His main arch-nemesis is Lord Voldemort (also known as "He-Who-Must-Not-Be-Named"). Their rivalry is a central theme throughout the series.
* Harry Dresden: This Harry is from the "Dresden Files" series by Jim Butcher. While he faces many adversaries, his main nemesis is likely "The White Court", a powerful group of vampires.
* Harry Truman: This Harry was the 33rd President of the United States. His main political adversaries would depend on the period, but the most prominent was likely Joseph Stalin during the Cold War.
* Other Harrys: There are many other fictional Harrys, each with their own potential arch-enemies.
Can you tell me which Harry you're thinking of so I can give you a more specific answer?