Special Agent Caitlin "Kate" Todd is a fictional character in the CBS drama NCIS, and was portrayed by actress Sasha Alexander. She was introduced in the show's first episode "Yankee White", replacing Robyn Lively's character Vivian Blackadder, who had appeared in the JAG Season 8 episodes "Ice Queen" and "Meltdown." Blackadder was written out of the show after test audiences viewing the two episodes reacted very negatively to her, resulting in the character of Kate Todd being created when NCIS went into regular production. First encountered by the team as a United States Secret Service special agent assigned to the Presidential Protection Detail on Air Force One, she was promptly recruited by senior special agent Leroy Jethro Gibbs at the end of the first episode following her resignation from the USSS for breaking the rules about not dating a colleague or having a relationship with them. She quickly became a key member of the Gibbs-led Major Case Response Team based at the NCIS headquarters in Washington, DC.