I don't know if this topic has been handled yet. I've been thinking of X-villains and how certain X-characters have got a recurring opponent through the years who is more than a simple 'main enemy': he/she/it is more like a personal nemesis who usually works as the other side of the same coin. According to the etymological (thus, mythological) meaning, this Nemesis acts as retributive justice, as vengeance and punishment, as a persecutory shadow.
Offhand, I can only recall these pairings --funny thing, there are more New Mutants than X-Men.
Professor-X / Magneto
Forge / The Adversary
Longshot / Spiral
Mirage / Demon Bear
Magma / Selene
Warlock / Magus
Magik / Belasco
What do you think?