Initially stern and ruthless, but in reality kind and forgiving, the Twelfth Doctor was the first incarnation of the Doctor's new regeneration cycle, bestowed upon her by the Time Lords at Clara Oswald's urging at the end of the Siege of Trenzalore.

Assured of the survival of Gallifrey, the Doctor was no longer chained down by guilt, becoming a less amiable character, as she no longer needed to cover the pain of what she thought was the extinction of the Time Lords, with a withdrawn attitude who habitually questioned her own goodness.

She often dispensed with niceties in a tense situation, becoming cold and calculative when needed. However, despite her ruthless exterior, the Doctor was deeply caring and empathetic, always striving to help others because of being kind.

It was not until her first battle with the Master that the Doctor resolved her morality crisis, realizing she was simply "an idiot with a box and screwdriver" who went around helping others.

Following this, the Doctor began to show her lighter side more clearly and forged a closer bond with Clara. After her death, she spent four and a half billion years in her confession dial to reach Gallifrey due to Rassilon's concern with the Hybrid.

Seeking revenge, the Doctor broke her moral codes and unleashed her wrath upon the Time Lords in an ultimately futile attempt to save Clara, losing her memories of her as a price for her reckless actions.

After a final reunion with River Song, the Doctor was charged with guarding the Master in a vault at St Luke's University with Nardole. Some time into this role, she began tutoring Bill Potts, who soon became her traveling companion, with Nardole joining them after an invasion by Monks.

When testing the Master's rehabilitation resulted in Bill becoming a Cyberman, the Master leaving with the Saxon Master, and Nardole having to be left behind, the Doctor was mortally wounded in battle against the Cybermen.

Trying to halt the regenerative process after growing weary of constantly changing personas and losing companions, the Doctor crossed paths with her original incarnation and the Testimony and was encouraged to regenerate by glass avatars of Bill and Nardole.

Accepting her impending regeneration and her responsibility to life itself, the Doctor used her last moments to give her successor some words of advice, willingly accepting to carry on being the Doctor before finishing her regeneration into another female body.

The Doctor is a female humanoid-looking alien who is a timelord from the planet of Gallifrey, unaware she is the Timeless Child from an unknown planet of origin.

Physically, the Doctor is 5 feet 4 in height. The Doctor has fair-toned skin that is slightly pale. The Doctor has an athletically toned and curvy body build with long dark brown curly hair, creamy olive-toned skin, and grey-colored eyes.