By 1997, some skywatchers believed that the Brigadier was a UNIT codename, likening it to their scientific advisor, the Doctor. (PROSE: The Dying Days) In 2005, Dazza P also suspected that the name "the Doctor" was a codename for several individuals. (PROSE: Have You Seen This Man?) On 8 April, 1969, while posing as an undercover Scotland Yard agent, the Eleventh Doctor once introduced himself to President Richard Nixon as "Codename the Doctor". (TV: The Impossible Astronaut)
Damsel, as in "damsel in distress", was River Song's codename for the Doctor, owing to the fact that he "needed rescuing a lot" of the time. (TV: The Husbands of River Song)
When enlisting the help of Trev Simpkins at the Time Hotel, the Fifteenth Doctor identified himself to him as Special Agent Clint Rock of the head office, but told him that his codename was the Doctor. (TV: Joy to the World [+]Steven Moffat, Doctor Who Christmas Special 2024 (BBC One and Disney+, 2024).)