Secret

Secrets were knowledge that an individual, or group of, withheld from others.

The Doctor
The Doctor had many secrets. The existence of War Doctor was thought to be the secret that the Doctor would take to his grave. (TV: The Name of the Doctor) The Eleventh Doctor later noted that he had kept the War Doctor a secret, even from himself. (TV: The Day of the Doctor) The same Doctor told Akhaten that he knew secrets that must never be spoken. (TV: The Rings of Akhaten)

In an alternate timeline, the Doctor defended his habit of keeping secrets to Clara Oswald as keeping his companion safe. Before he reset the timeline, and upon learning that Clara Oswald had read his true name in The History of the Time War, he made her promise to not go looking for his secret. (TV: Journey to the Centre of the TARDIS)

Martha Jones initially thought the Master to be the Doctor's "secret brother", the Tenth Doctor waving off the idea as her having watched too much television. (TV: The Sound of Drums)

In his days at the Time Lord Academy, the First Doctor became trapped in the Cloisters, being told of a secret way out by the Sliders. (TV: Hell Bent)

The Twelfth Doctor described his knowledge of the Hybrid as a "very, very dangerous secret", (TV: Heaven Sent) though a Time Lord author believed that the Doctor was lying about having knowledge of the Hybrid, using the Time Lords' fear of it against them as revenge for their killing Clara Oswald. (PROSE: A Brief History of Time Lords)

Lady Peinforte described the TARDIS Wiki as a "place of secrets", in which she cribbed from to learn of the Doctor's secrets. (PROSE: Lady Peinforte)

Yasmin Khan noted the Thirteenth Doctor to be very secretive about her past. (PROSE: Chasing the Dawn)

Others
George Thompson kept many of his fears a secret from his parents, instead writing about them in a special book. (PROSE: My Special Book)

When acting as a governess for Francesca and Digby Latimer, Clara Oswin Oswald affected an upper-class accent, her true working class one being known to the children as her "secret" voice. (TV: The Snowmen)

When describing the Time Lords to Jamie McCrimmon and Zoe Heriot, the Second Doctor noted that his people had "the secret" of space/time travel. When the Doctor was later exiled, the First Time Lord noted that "the secret of the TARDIS" would be taken from him. (TV: The War Games) The Master referred to the Death Zone as the "black secret" at the heart of the Time Lords' paradise. (TV: The Five Doctors)

Information from invalid sources
In one million BC, the First Doctor told the Tribe of Gum that he was unable to grant them the secret of fire but he was able to present them with something else that would warm them up a bit. This turned out to be Raquel Welch. (NOTVALID: An Unearthly Child: The Unscreened Edition)

The Evening Standard once reported on a "Cyberleader's sordid love secret" in a front-page newspaper article by Sarah Jane Smith. (NOTVALID: Doctor Who? 163)