Lilia Flimlight

Lilia Flimlight is the protagonist of Baphomet Complex, one of the Monster Girl Encyclopedia Stories novellas. A very (very) petite sixteen-year-old human, she is transformed into a baphomet monstergirl by an itinerant lilim, and proceeds to rape her older brother for ten days straight.

Baphomets aren't that complex
Lilia features only in Baphomet Complex, although since this constitutes 60-pages of first-person-perspective it makes her one of the most thoroughly described characters in the Monster Girl Encyclopedia world:

Lilia's only family is her older brother, with whom she is deeply and secretly infatuated. Together they run "Flimlight's Odds and Ends", a shop in an un-named anti-monster state which stocks gear for adventuring parties. Although she's technically the co-owner, it is in practice Lilia's brother who actually manages the business, with his little sister consigned to just being the shopkeeper. In this capacity she leads quite a boring existence: dozing behind the counter waiting in vain for customers to appear, and fretting at how her under-matured looks stand no chance of captivating the object of her affections.

Like all good godfearing folk in the anti-monster states, Lilia is frozen with "unspeakable terror" when a lilim appears in her store. Somehow sensing that Lilia is one of those rare humans who can be transformed into a high-powered monstergirl, the daughter of the Demon Lord can't pass up the opportunity for sexual mischief: "Since human women who can be changed into these races are extremely precious, when a lilim spots these women, she will aggressively attempt to change them into these monsters . In spite of Lilia's screams of protest, demonic energy violates her completely.

Initially greeting her transformation with horror and despair, Lilia is goaded by the lilim to go rape her sleeping brother. After doing precisely that, "so much so that the first several days we'd even forgotten eating and sleeping", the neophyte baphomet and her now-incubus brother are revisited by the lilim, and instructed to establish a branch of the Sabbath in the town.