Who was replaced by Cote de Pablo?

Answer 1

Special Agent Kate Todd (Sasha Alexander) was killed in the season 2 final episode Twilight. She was killed by Ari Haswari (A rogue agent of the Israili Mossad (Israili Secret Service).

Agent Ziva David (Cote de Pablo) who was to be her replacement, first appeared in the episode Kill Ari part 1, which was the first episode of the third season. She is an agent of the Mossad assigned to Gibbs team by the new NCIS director Jenny Shepherd to prevent Gibbs from killing her fellow Mossad officer and half brother Ari Haswari (even though he was guilty of killing an NCIS agent).

In the 4th episode of Season 3 (Silver War), NCIS Director Jenny Shepherd officially gives Ziva a permanent position on Gibbs team as Mossad Liason Officer.