Day 3: 11:00 P.M.-12:00 A.M. - S3-E11
Trivia: When the CTU personnel are in a meeting and Adam brings a picture of Nina onto the big screen, the name next to the picture is Sarah Berkeley, which is Sarah Clarke's married name. Clarke has been married to Xander Berkeley since 2002.
Answer: According to Jack in season 5, Nina was "deep cover" before the pair ever met, so it's safe to assume she was recruited before she joined CTU. As for what her primary mission was, who knows? Perhaps it was to gather sensitive intel and pass it on to her superiors. Given that she worked for an intelligence agency, it's not a stretch. Or perhaps we may find out in season seven.
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