An analyst at CTU Los Angeles (and later New York), she is one of Jack Bauer's most trusted colleagues, often doing unconventional and unauthorized favors for him, even at personal risk to herself. As O'Brian, Rajskub appeared in 125 episodes of 24, more than any other actor except series star Kiefer Sutherland, who appeared in all 192 episodes of the series.
She is exceptionally intelligent; in particular she displays extraordinary mastery of computer science. Spending most of her time behind a computer terminal, she is rarely sent on field assignments; however, she has demonstrated proficiency with weapons in Day 4, 5, and 8. She works very well under pressure, yet it is obvious when she is under pressure, and she has always demonstrated a degree of social awkwardness.