23. Nina Meyers, 24
Played By: Sarah Clarke
It's quite an ordeal for a show's hero and the audience alike when our protagonist's staunchest allies become his or her greatest enemies. Shortly before the events of 24 begin, Jack Bauer (Kiefer Sutherland) is working with Nina Meyers by day and sleeping with her at night. If film and television have taught us anything, it's that inter-office fraternization isn't advisable, particularly if your line of work is government super-spy games.
Predictably, their working relationship quickly turns sour, creating a complex cat-and-mouth back and forth that's as riveting as any double- and triple-crossing sexually tense spy story arc we've ever seen.