Caitlin 'Kate' todd
Caitlin Todd, portrayed by Sasha Alexander, first appeared in the episode "Yankee White". Todd was a former Secret Service agent, recruited by Gibbs after she successfully helped him solve a murder aboard Air Force One. She worked well with everyone on the team, becoming particularly close with Ducky and Abby, who convinced her to get a tattoo (referenced in the episode "Kill Ari (Part 1)"). Her relationship with Gibbs is unique, as there appeared to be a real friendship between the two characters; which is unusual considering Gibbs is not a close friend with anyone. Her relationship with Tony, however, was more adversarial and appeared to be more of siblings than anything else. Tony frequently flirted with her and went through her personal belongings, no matter how many times she pointed out that his behavior was grossly unprofessional. At the same time, Kate is willing to risk her life for DiNozzo and admits that life would be considerably less interesting without him around.
Kate was killed in the line of duty at the end of the episode "Twilight" by Ari Haswari, collateral damage in the terrorist's obsession with Gibbs. Todd also made appearances in "Kill Ari" parts 1 and 2 as a hallucination, remembered by her teammates. She was replaced at NCIS by Israeli Mossad Liaison Officer Ziva David.