Question: what episode does ziva join ncis?
Answer: Ziva first appeared in the season 3 premiere episode, "Kill Ari (Part 1)", and became a regular cast member from the episode "Silver War". She replaced Caitlin "Kate" Todd (Sasha Alexander), who was killed at the hands of Ziva's half-brother, Ari Haswari (Rudolf Martin), in the season 2 finale.