The Lumiat

Tired of her dark ways, and hoping to impress the Doctor, the dying Missy created the Elysian field to 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
The Lumiat foiled several of her immediate predecessor's schemes, causing Missy to believe the Lumiat was the Doctor. After revealing her 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. However, she was mortally wounded by Missy after she "[grew] bored" of her, and was forced to regenerate, with Missy speculating that the next incarnation would not share the Lumiat's good nature. She subsequently dumped her on a random planet, where she completed her regeneration, screaming in pain, in view of some baffled villagers. (AUDIO: The Lumiat)

The Lumiat, hoping to ensure her own existence, projected herself from the future into an aborted timeline to warn both Missy and Jo about the consequences of the Master's actions. She exhorts Jo to trust her because she incarnates the good side of the Master, but Jo retorts the Master has always been capable of goodness. The Lumiat disappears, however, before she can 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 their 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 on the role were known.