Fugitive Doctor

An incarnation of the Doctor, identified as such by the Judoon and the Thirteenth Doctor's sonic screwdriver, had a history of working for officials on Gallifrey. She served in a role she had not signed up for, and which she sought, though failed, to abandon. At one point, the Doctor hid out on Earth using a Chameleon Arch, taking on a human identity as "Ruth Clayton".

She had her TARDIS buried near the lighthouse where she stored her true identity, and trusted Lee Clayton to protect her, under cover as Ruth's husband. Lee was also tasked with reminding her to "Follow the light and break the glass" when the time was right, as this would lead Ruth to return to the lighthouse, which she was made to recall from childhood, and finally restore her Time Lord memories. (TV: Fugitive of the Judoon)

Working for the Time Lords
For a period of time, having been consigned or recruited, the Doctor worked for the Time Lords. She tried to flee from this job on at least one occasion, but learned that, now she was involved, escape would not prove an easy task. Later on, she would characterise this role as "one you can [never] leave".

At one point, she fled to Earth and used a Chameleon Arch to disguise herself as a human, with Lee Clayton acting as her protector in case of trouble. (TV: Fugitive of the Judoon)

As Ruth Clayton
Taking up residence in Gloucester with her "husband", Lee, Ruth Clayton made a living as a self-employed tour guide. She believed herself to be human, now 44 years old, and remembered being brought up by her parents in a disused lighthouse.

According to her constructed memories, Ruth moved away from home, seeking a life of her own. Her parents died after this point, and Ruth inherited the lighthouse from them, though she did not wish to live there. According to Ruth, she moved to Gloucester in mid-December 1999. She did not revisit her childhood home, since the death of her parents, until meeting the Thirteenth Doctor. (TV: Fugitive of the Judoon)

By 2020, (TV: Can You Hear Me?) she was a regular customer at a coffee shop where she had become friendly with the barista, Allan. Allan tried to convince Ruth that Lee was not good enough for her, even keeping a dossier which recorded his suspicions regarding him.

When Team TARDIS arrived, the Judoon, under contract with the Time Lords, had pinned down the Doctor and Lee's general location, in Gloucester, due to Lee's sentimentality in keeping an old service medal. Both witnessed the Judoon's arrival. At home, Lee told a dismayed Ruth that they urgently needed to pack up and go, without explanation. Lee distracted the Judoon, allowing Ruth to escape with the Doctor, but sent her a cryptic text message, and was ultimately killed by Gat.

As her true self began to awaken, due to Lee's message to "Follow the light and break the glass", Ruth lost control of her own body, and entered into physical combat with the Judoon. In an act beyond self-defence, Ruth pulled off Captain Pol-Kon-Don's horn, causing the Thirteenth Doctor to inquire after her true identity.

As memories began surfacing unexpectedly, she and the Doctor then travelled to the lighthouse, where she followed her imperative to "break the glass". After shattering the biodata module with the push of a button, the Doctor's Time Lord personality was reactivated. (TV: Fugitive of the Judoon)

Memories restored
With her true self restored, she retrieved a rifle belonging to Gat, quickly changed into her old clothes, and went outside to introduce herself to the Thirteenth Doctor, who had dug up the other Doctor's TARDIS. The "fugitive" Doctor then teleported the two of them onboard, where she learned of her companion's identity. Since she didn't recognise the Thirteenth Doctor, she assumed she was encountering a future incarnation of herself, only to learn that the Thirteenth Doctor had no memory of ever being her, either.

Before either Doctor could figure out what was going on, the Doctor's TARDIS was captured by the Judoon. Onboard the Judoon ship, the two Doctors encountered Gat; the Doctor returned her rifle to her. She attempted to pass off her other incarnation as "nobody", but the Thirteenth Doctor ultimately revealed who she was. To the Doctor's surprise, the Thirteenth Doctor also claimed that Gallifrey had been destroyed, sharing her memories of this with Gat as proof that she was not lying.

Gat tried to execute both Doctors, despite the Doctor's pleas for her to stand down. Having sabotaged the rifle, the Doctor knew that it would backfire on Gat, killing her. Setting aside the Thirteenth Doctor's horror at her actions, the Doctor retrieved the firearm and reversed her alterations. Using this weapon, she threatened the Judoon with it, pointing out that, now they were in interstellar space, crime was not defined, and they had no jurisdiction. After the Judoon let them go, the Doctor returned her other self to Earth, noting that they couldn't both be right about each other's place in their chronology. (TV: Fugitive of the Judoon)

Personality
The Doctor had a low opinion on the Thirteenth Doctor's sonic screwdriver, terming it a "gizmo", and arrogantly claimed she was too smart to use one for herself. (TV: Fugitive of the Judoon)

She exhibited a more violent persona than many of her other incarnations, threatening to kill a Judoon platoon, and arming herself with a gun as well as programming Gat's gun to blow its owner up. This behaviour may be viewed with some mitigation: there was an implication that the threat to kill the Judoon was a bluff, since she acknowledged that "the Doctor never uses weapons", and she warned Gat not to use the gun. (TV: Fugitive of the Judoon)

She also showed dislike for the Thirteenth Doctor. After instructing her to keep quiet and let her do all the talking, the Doctor was furious with her for interfering in her plans for Gat. (TV: Fugitive of the Judoon)

Habits and quirks
This Doctor spoke in a traditional London accent, both while under the impression of being Ruth Clayton, and as her true self.

In a manner similar to the Thirteenth Doctor, she gives out points as people got something right. (TV: Fugitive of the Judoon)

Skills
The Doctor had the combat skills to overpower a platoon of Judoon with her bare hands, and even had the strength to remove Captain Pol-Kon-Don's horn with one hand. (TV: Fugitive of the Judoon)

Appearance
This incarnation resembled a dark-skinned middle-aged woman. She had black hair, which she wore in dreadlocks. (TV: Fugitive of the Judoon)

Clothing
The Doctor wore a royal blue tweed frockcoat with 14 silver buttons. She wore a kente shirt under a double-breasted waistcoat of navy blue moleskin with 16 bronze buttons. She also wore black trousers and boots. (TV: Fugitive of the Judoon)

Behind the scenes
While Fugitive of the Judoon left it unclear where this incarnation of the Doctor fits into the Doctor's life, Chris Chibnall confirmed to The Mirror that, as stated in the episode, "She is definitively the Doctor." He went on to say that "There's not a sort of parallel universe going on; there's no tricks."

First POC to play the Doctor?
This incarnation of the Doctor was the first within the DWU to be played by a non-white actor, Jo Martin. Some stories have previously shown the Doctor briefly taking over the bodies of non-white humans, and so getting played by those who were cast for those characters, and others have already indicated that non-white incarnations do in fact exist somewhere along the Doctor's timeline.

Daniel Anthony played the Eleventh Doctor in The Sarah Jane Adventures story Death of the Doctor while he occupied the body of Clyde Langer, and Damian Lynch played the Third Doctor in the audio story Ghost in the Machine while he was using the body of Benjamin Chikoto to communicate.

More concretely, Russell T Davies' Rose novelisation shows that Clive Finch has collected a photograph of, among other known incarnations, a tall, bald, black female Doctor who wields a flaming sword.

In 1986, Lenny Henry also portrayed a parodic incarnation of the Doctor in a comedy sketch as part of his BBC series The Lenny Henry Show.