Vivienne Blackadder is in the two "JAG" episodes
"Ice Queen" and "Meltdown". She was FBI before her brother Rex died in the terrorist attack on the
USS Cole. They were close; he taught her to track and called her "Pocahontas." Beause of his death, she left the FBI to join NCIS. When NCIS captures one of the terrorists behind the Cole attack, she wants in on questioning, but Gibbs warns her that NCIS is not her tool to avenge her brother.
When Blackadder officially joined NCIS is up in the air. The attack on the Cole was October 2000, so if she joined any time in the next year (through October 2001), she would have joined NCIS even before Tony did (he joined October 2001 ["The Curse"]. It could be that she either joined NCIS early and later joined Gibbs' team or that she joined NCIS after Tony and was added to Gibbs' team immediately. Tony's comment that she "hooked up with us" and Gibbs' inclination to put Tony in charge seems to indicate that, at any rate, Tony was on the team before Viv.