Twelfth Doctor (BBC)

Stern, ruthless, but also kind and forgiving, the Twelfth Doctor was the first incarnation of the Doctor's second regeneration cycle, bestowed upon him by the Time Lords at Clara Oswald's urging.

Assured of the survival of Gallifrey, the Doctor was no longer chained down by guilt, becoming a less amiable character, as he no longer needed to cover the pain of what he thought was the extinction of the Time Lords, with a withdrawn attitude who habitually questioned his own goodness. He often dispensed with niceties in a tense situation, becoming cold and calculative when needed. However, despite his ruthless exterior, the Doctor was actually deeply caring and empathetic, always striving to help others for the sake of being kind.

It was not until his first battle with Missy that the Doctor resolved his morality crisis, realising he was simply "an idiot with a box and screwdriver" who went around helping others. Following this, the Doctor began to show his lighter side more clearly and forged a closer bond with Clara. After her death, he spent four and a half billion years in his confession dial to reach Gallifrey due to Rassilon's concern with the Hybrid. Seeking revenge, the Doctor broke his own moral codes and unleashed his wrath upon the Time Lords in an ultimately futile attempt to save Clara, losing his memories of her as a price for his reckless actions.

After a final reunion with River Song, the Doctor was charged with guarding Missy in a vault at St Luke's University with Nardole. Sometime into this role, he began tutoring Bill Potts, who soon became his traveling companion, with Nardole joining them after an invasion by Monks. When testing Missy's rehabilitation resulted in Bill becoming a Cyberman, Missy leaving with the 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 his original incarnation and the Testimony, and was encouraged to regenerate by glass avatars of Bill and Nardole. Accepting his impending regeneration and his responsibility to life itself, the Doctor used his last moments to give his successor some words of advice, willingly accepting to carry on being the Doctor before finishing his regeneration into a female body.