Caitlin Todd first appears in the episode "Yankee White". Todd is a former Secret Service agent, recruited by Gibbs after she successfully helped him solve a murder aboard Air Force One. She works well with everyone on the team, becoming particularly close with Ducky and Abby, who convinces her to get a tattoo (referenced in the episode "Kill Ari (Part 1)"). Her relationship with Gibbs is unique, as there appears to be a real friendship between the two characters, which is unusual considering Gibbs is not a close friend with anyone.
Todd's relationship with Tony, however, is more adversarial and appears to be more like siblings than anything else. Tony frequently flirts with her and would sometimes go through her personal belongings, no matter how many times she points out that his behavior is 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.

Following Sasha Alexander's decision to leave the show, Todd is 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. She dies after being shot in the head by a lone bullet that had been fired from a model of sniper rifle (Bravo 51) nicknamed "Kate", which had been manned by Ari himself. She is replaced at NCIS by Ziva David.