"Nari" was an alias taken on by an alternative personality crafted for herself by a renegade of the Great Houses during the War in Heaven.
Biography
Original Homeworlder
As her original self, the Homeworlder was familiar with Dionus during their early lives on the Homeworld. She was a talented chemist. However, she became an infamous criminal, among the Homeworld's "chief renegades". At some point during the War, the authorities began to actively look for her. To evade detection, she used her knowledge of brain chemistry on herself to craft an innocent alternative personality, "Nari", and bury her true, "evil" self beneath it. Nari was unable to access her original self's memories of her time as a renegade, passing out whenever she attempted to do so, although she did remember her youth on the Homeworld and was aware of her true nature in an abstract sense. (AUDIO: Me & My Ghost)
Nari deceives Dionus
Eventually, however, Nari realised that she had done too good a job and that she could not unlock her true personality no matter how hard she tried. Dionus had, by then, defected from the House Military and opened a clinic on the planet of Gulliver's Rest, on the fringes of the War. Using artificial pheromones to make him like and trust her instantly, she deposited herself at his doorstep, claiming to simply have amnesia. He took her in as a patient and became infatuated with her, leaving her unsupervised at night. On her second night at the clinic, in her sleep, her original personality briefly broke through and she brutally murdered another patient before the "Nari" personality became dominant again, her hands still bloody but with no memory of what she had done. (AUDIO: Me & My Ghost)
Discovery and disappearance
After locking her up for the remainder of the night, Dionus realised who she really was by hacking into the Great Houses' mainframe, and interrogated her. However, when he tried to get her to tell him why she'd come to the clinic, she passed out. Hooking her up to one of his old interrogation devices while she was unconscious, he realised the truth about the Nari personality, as well as the fact that she had been essentially drugging him with the pheromones.
After she awakened, he made a deal with her, keeping her locked up but promising to work on a way to free her real personality if, in the meantime, she put her talents as a chemist to use helping Dionus with his work as a physician. Nari agreed, and stayed at the clinic for some time in this arrangement, with Dionus coming to genuinely enjoy her company with no need for pheromones. Ultimately, he let his guard down to the point of allowing Nari to cook for a shared dinner they planned to have; Nari drugged his food and disappeared while Dionus was unconscious. He did not expect to meet her again, but bore her no grudge, happy that she was still alive somewhere and hoping that her time at the clinic, however unorthodox, had still done her some good. (AUDIO: Me & My Ghost)
Behind the scenes
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Bill Baggs's intention writing Me & My Ghost was obviously that the renegade who turned herself into "Nari" was the Rani: the name Nari, taken from a minor, male character from Norse Mythology, is an anagram for her title, and she is noted to have talents as a chemist, mirroring the Rani's status as a "brilliant biochemist".
On the cover of Me & My Ghost created by Phil Shaw, Nari appeared as a mere shadow in the background, obviating the need to depict her facial features. The shadowy Nari's silhouette broadly resembled that of Kate O'Mara's version of the Rani, although it is not clear that Nari would actually have been intended to be the Kate O'Mara incarnation of the Rani: Dionus does not quite recognise her at first glance, despite feeling he has met her before, and Big Finish's Doctor Who audio plays had previously established that O'Mara's Rani regenerated into a new incarnation long before the Last Great Time War.