It has been confirmed that Linda Hunt's character Hetty is leaving the show. This saddens many fans but rumor has it that Claire Forlani (Peyton Driscoll - CSI:NY) may be filling her spot on NCIS Los Angeles.
As of September 2011, Linda Hunt is not expected to leave NCIS LA.
Kate was replaced by Ziva David in season 3 as a Mossad liaison officer in "Silver War" (3x04) but appeared in "Kill Air Part 1 and 2" at the beginning of season 3 (3x01 and 3x02).
It has been rumored that Linda Hunt's character, Heddi, is leaving the show. No details are known.
Washington DC
No. Kate was just played by one person - Sasha Alexander
Todd
We never found out on the show.
Wendy Makkena
No
No. She died.
Washington DC
No. Kate was just played by one person - Sasha Alexander
Todd
Gibbs and Gearld
We never found out on the show.
Wendy Makkena
She was awarded the Presidential Medal of Freedom posthumously
The episode mentioned above is Bete Noir, which was aired in Season One of NCIS.
Kate Todd is played by Sasha Alexander, who left the season for personal reasons. Her character, Kate, is an ex-Secret Service agent who was invited to join NCIS by Agent Gibbs after she disobeyed a superior. She did not leave for personal reasons, she died, shot by Ari, Ziva's half-brother.
Yes, Sasha Alexander plays Agent Todd in the first two seasons of NCIS.