Arch nemesis are friends from a long time ago that have more or less equivalent powers, but also have oposing ideologies. They are therefore always fighting with each other. Examples:
- Obi Wan and Darth Vader
- The Republican and Democratic Parties
Professor Xavier and Magneto are eachother's arch-nemesis. They play chess together and have been friends for a long time, but they completely disagree about the relationship that mutants should have with humans, so they are always fighting.