Rei (レイ) is one of the two main antagonists of a short story arc in the erotic doujinshi "Rougetsu Toshi" (朧月都市), aka. "Misty Moon Metropolis" released in 2000 and 2001. The story's artist is Kagerou1991.
Rougetsu Toshi XII: Chapter 1 - "Ritual". Rei, along with her companion Mutsu, assume the role of teachers at Nashi Academy by unknown means. There, they infect the students (all of them female) in her class with parasites. The parasites assess the strength of each of their hosts and give the stronger ones male genitalia so that they, at Mutsu's command, can rape the weaker ones.
Rougetsu Toshi XIII: Chapter 1.5 - "Aborticide". The plan of the two demons is foiled and Mutsu is killed. Knowing that she could well be next, Rei hides herself in the body of a girl named Erika.
Rougetsu Toshi XIII: Chapter 2 - "Erika Part 1". Erika - a girl born from a human and a demon - is taken to the home of series protagonist Suzuka. One night while Suzuka is away, Erika's demonic lust takes over and she rapes and brutally murders one of Suzuka's lady friends. It is here that a delighted Rei finally emerges from Erika's body, quite pleased with the carnage that has been created. Rei decides to thank Erika by forcing herself sexually onto the girl, but is forced to retreat once more when Suzuka returns home.
Rougetsu Toshi Gaiden Ichi: Chapter 3 - "Erika Part 2". A flashback reveals that Mutsu and Rei were originally both evil spirits who befriended Erika as a child by taking possession of one of her dolls and a teddy bear respectively. Knowing of Erika's demon blood, the "toys" eventually persuade her to remove the protective charms that her grandmother had placed around their home, so that the three of them can have some "fun". This act allows Mutsu and Rei to assume more humanlike forms. When Erika's grandmother arrives home, Mutsu murders her, and the two demons keep Erika as an enslaved source of sexual energy up until the events of Chapter 1/1.5.
Suzuka attempts to purify Erika by absorbing her powers and, unknowingly, draws Rei into her own body in the process. Rei appears once more shortly after and attempts to rape Suzuka, after seeing that she has the ability to produce a male member like Erika. Unfortunately for the villainess, Suzuka's power is far beyond that of the demon's, who then finds herself on the reverse end of the sexual assault. A highly aroused Erika catches the two and is invited by Suzuka to join in. Rei is unable to endure the power of the two females and ultimately explodes upon their climaxes, leaving behind only the mutilated teddy bear she had possessed long ago.
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Full gallery: Rei (Rougetsu Toshi)