The Lumiat

Tired of her dark ways, and hoping to impress the Doctor, the dying Missy created the Elysian field to artificially regenerate her body into a distillation of all her goodness. The regenerated Missy, inspired by the Doctor, went out to do good in the universe and abandoned her previous identity as the Master to go under the name "the Lumiat".

Biography
Mortally wounded by her own younger self and unable to regenerate, (TV: The Doctor Falls) Missy was forced to use an elysian field to survive, which kickstarted a new regeneration cycle by breaking the body down into atoms and then rebuilding it. As a side effect of this process, Missy's personality was altered, creating a new incarnation who was the distillation of her goodness; an act which the new incarnation believed had been deliberate, though a younger Missy disputed this claim. The new incarnation decided to adopt the title of the Lumiat and found many of her memories were missing as a result of the process.

The Lumiat foiled several of her immediate predecessor's schemes, including reversing the effect of an atmospheric temperance field Missy had used to make a group of people having a picnic fight each other, using Cybercats to kill flesh-eating rats Missy had unleashed on the Infinity Prairies and stopping fragments of an asteroid Missy had detonated impacting a nearby spaceship, causing Missy to initially believe the Lumiat was an incarnation of the Doctor. After revealing her true identity, the Lumiat accompanied Missy on some trips, trying to act as a good influence on her. She also stole Missy's newly-rebuilt Tissue Compression Eliminator.

The Lumiat finally lost her patience with Missy after she threatened to destroy a Siren Gull egg, the discovery of which had led to a truce in a civil war. When she refused to put it down, the Lumiat fired at her with the TCE but missed, and was then shot by Missy whilst trying to catch the egg after she threw it. Deciding she'd become bored of her, Missy subsequently dumped the Lumiat on the verge of regeneration on a random planet and speculated that the next incarnation would not share the Lumiat's good nature. The Lumiat completed her regeneration, screaming in pain, in view of some baffled villagers. (AUDIO: The Lumiat)

Aborted timeline
In an aborted timeline the Lumiat projected herself from the future to warn both Missy and Jo Grant about the consequences of the Saxon Master's actions whilst they explored the ruins of Kiameth. She exhorted Jo to trust her because she incarnated the good side of the Master, but Jo retorted the Master had always been capable of goodness. The Lumiat disappeared, however, before she could explain herself. (AUDIO: Masterful)

Appearance
The Lumiat was a middle-aged woman with collar-length auburn hair and blue eyes. She wore a white collarless blouse. (AUDIO: The Lumiat)

Psychological profile
Due to her previous incarnation's use of an Elysian field to regenerate, the Lumiat claimed to be a distillation of all her past self's positive qualities. She cherished life, weeping over the death of Bertram, a companion Missy had killed after becoming bored of him.

The Lumiat showed disgust for her past selves' actions, deriding some of their failed schemes, though regarded her predecessor's ultimate fate as "poetic". By contrast, she admired the Doctor, claiming to be a "fan of her work", and was flattered that Missy had initially mistaken her for them.

Despite claiming to have completely abandoned her old ways, the Lumiat did resort to threatening Missy with her TCE after her patience had worn thin. (AUDIO: The Lumiat) She also displayed a somewhat manipulative side, encouraging Missy to give up her life so she could come into existence earlier. (AUDIO: Masterful)

Behind the scenes

 * The ending of The Lumiat seemingly implied that the Lumiat would regenerate into the Spy Master, who made his debut on television shortly before the release of the story. This was a coincidence, as the script was written and recorded long before the plans for Sacha Dhawan to take over.