The CTU Chief of Staff is the second-in-command of a CTU Domestic Unit. Often called Deputy Director, or simply "Number Two". If the Special Agent in Charge of a domestic unit is not available or unfit for command, the Chief of Staff assumes his responsibilities.
CTU Los Angeles
The following is a list of Chiefs of Staff at CTU Los Angeles.
As Jack's closest ally, Nina Myers served as Chief of Staff at CTU Los Angeles during Day 1. When she was exposed as a mole, Nina was fired and charged with treason and the murder of Teri Bauer.
After Nina Myers was removed, Tony Almeida assumed the position, serving under George Mason. When Mason stepped down due to radiation exposure, Tony was promoted to Special Agent in Charge.
When Tony Almeida was promoted to Special Agent in Charge during Day 2, Michelle got promoted to Chief of Staff. She held the position through most of Day 3. However, when Tony was shot in the neck, Michelle was become acting Special Agent in Charge. After the events of Day 3, Michelle left CTU Los Angeles for CTU Seattle and eventually received a position at Division.
Nadia Yassir became the Chief of Staff after the events of Day 5. Serving under Bill Buchanan, Nadia lost most of her access at CTU when she was flagged by the Department of Homeland Security. When Buchanan was forced down, he named Nadia acting Director of CTU in the interim.
Jack Bauer fills a role similar to this once Larry Moss was killed and CTU was reestablished in the FBI's headquarters in Washington, D.C. with Agent Renee Walker assuming command as FBI director for the time being.
It is unknown who the Chief of Staff is at CTU New York during Day 8, or if the role even exists after the re-establishment of the agency.