The Nun

One incarnation of the renegade Time Lord often dubbed "the Monk" embraced the spirit of the name, taking on the title of "the Nun" due to having regenerated into a female form. She faced off against the Tenth Doctor, who had accidentally drawn himself into the time period before the Last Great Time War as an unintended result of the Nun's scheme to modify history.

The Nun was uncertain of how many regenerations she had gone through, as her chronology was complicated by her meddling with her own past, erasing parts of her history. (AUDIO: The Wrong Woman)

Early days
After regenerating into a female incarnation, Mortimus, having been dubbed in previous male incarnations as "the Monk", chose to dub herself as "the Nun". (AUDIO: The Wrong Woman)

Releasing his past self from Missy
The Nun decided to help her past self escape from Missy's hold. To do so, she stole a battery of plutonium from an intergalactic warlord and brought it to Earth, in 16th century Italy; there she convinced a young servant girl, Francesca, to kill the warlord and pretend to be the duke of Rimini so to help her. She then contacted Missy and promised to give her the coordinates of her Master TARDIS, in exchange for the deliverance of his past self, which the Nun said she was going to torture. The two couples met at the villa of Alfredo Borgia in Tuscany, where the Nun repeatedly try and eventually succeeded to inform his past self of who she was. Together, they were eventually able to escape together in the Nun's TARDIS, leaving Missy stranded in Italy, and went on to meddle in history together. (AUDIO: Two Monks, One Mistress)

Impersonating the Tenth Doctor
She met George Sheldrake and helped him develop time travel, however he stole her TARDIS' temporal velocitor and attempted to get rid of her in the Vortex. She escaped and returned to Earth to reclaim her stolen component, using a psychic shroud to pose as a pastor in a failed attempt to gain access to Sheldrake's building and disguising her TARDIS as a holy book. (AUDIO: The Wrong Woman)

Whilst preaching outside Sheldrake's building, she encountered the Tenth Doctor and Space Security Service agents Anya Kingdom and Mark Seven who were investigating Sheldrake. She confronted the Doctor in the SSS headquarters and shot him, (AUDIO: Buying Time) stunning him so she could imprison him in her TARDIS. She then used the psychic shroud to impersonate him and pretended to regenerate into her normal appearance, convincing Anya and Mark she was the Doctor. She used their investigation to reach the velocitar and reclaimed it, spitefully telling Anya she didn’t forgive her knowing that she was seeking to make amends with the Doctor.

She escaped in her TARDIS, abandoning Anya to Sheldrake's security, and ignored the captive Doctor's warnings of temporal cataclysm. His warnings come true when Sheldrake activated her time tunnels without the velocitar, so the Nun reluctantly returned to Earth with the Doctor as her TARDIS was unable to move through the non-existent Vortex. They managed to stabilise the Vortex using Sheldrake’s equipment however the Nun chose to take the opportunity to flee in her TARDIS without helping the Doctor fix the situation entirely, fearing he would punish her afterwards. (AUDIO: The Wrong Woman)

Legacy
Romana II and the Fourth Doctor once mentioned "the Interfering Nun" among other renegades during a discussion of Time Lord who broke with traditional Gallifreyan inactivity. (PROSE: Shada)

Personality
Well-spoken and projecting a boisterous enthusiasm, the Nun had a slippery underside that she masked by presenting herself as a posh and well-to-do woman. (AUDIO: The Wrong Woman)

The Nun enjoyed meddling with history, believing she exceeded at thinking on her feet. She was also incredibly careless when it came to time travel, taking for granted that there would "always be time". (AUDIO: The Wrong Woman) Unlike other Time Lords, the Monk took little care of any potential damage to fixed points or to the Web of Time.

The Nun showed a childish and petulant side to her nature, taking pleasure in showcasing the complexity of her plans and making brash decisions and feeling she had no responsibility toward her own actions. The Nun was considerably more nervous and paranoid than previous incarnations, fearing that the Doctor would strand her on a frozen tundra (TV: The Daleks' Master Plan) or report her to the Time Lords for her misdeeds. This distrust and fear would ultimately trump any misgivings she had of her own intellect. (AUDIO: The Wrong Woman)

She considered the Academy to be very dull, and thus she never paid attention to any lectures. This would lead to her often placing herself in situations that were out-of-her-depth. While she did not trust the Doctor, believing that he would betray her at a moments notice, she understood that she knew less about the Laws of Time than her contemporaries. (AUDIO: The Wrong Woman)

She demonstrated a cruel streak, taking the opportunity to tell Anya Kingdom that the Doctor would never forgive her whilst impersonating him. She took great joy pretending to be the Doctor, deeming him "wacky" and "eccentric". (AUDIO: The Wrong Woman)

Appearance and clothing
The Nun, in this incarnation, resembled a woman in her mid-thirties, possessing jaw-length brunette hair and blue-coloured eyes.

The Nun typically wore a habit, as she told the Tenth Doctor that she "...so [suited] the look of a convent." When impersonating the Doctor, she initially wore an over-sized blue suit with red pinstripes. Under the suit, she wore a white dress shirt and a loose tie.

While working with Anya Kingdom and Mark Seven, she replaced the suit with a uniform she retrieved from the Space Security Service headquarters; an older-styled silver jumpsuit. Upon revealing her identity to the Tenth Doctor, the Nun discarded the jumpsuit in favour of her usual outfit. (AUDIO: The Wrong Woman)

Behind the scenes

 * According to cover artist Simon Holub, Gemma Whelan's face was omitted from the cover art for Dalek Universe 1 boxset to ensure that the cliffhanger ending of Buying Time was not spoiled.
 * In Doom Coalition 3, River Song, who is part-Time Lord, and is disguised as a nun, is referred to by the Clocksmith as "the Interfering Nun".
 * John Dorney and David Richardson initially wanted the Monk to appear in the storyline which eventually became Daughter of the Gods. They issued David K Barnes a list of characters from the early years of Doctor Who and tasked him with writing a story around them as if emulating a "five-year anniversary special" from 1968. However, while most of the characters on the list made it into the final work, the Monk was ultimately dropped once the main story began to take shape. (VOR 128)