It's someone who works against you at all times, seeking to foil your plans. They have purpose, and meaning, even. They eventually come to know all about you, and you know all about them, and you pay special attention to anything they do, and vice versa. Very romantic relationship. I should put an ad in the paper.
Protagonist seeks arch-enemy.
Must have high-tech surveillance equipment, lots of spare time and money, and some sort of scar or distinguishing mark. Secret base should be in Seattle area. No sidekicks, please. I'm taking my lenses out now. It's harder to stay awake with them out.