"You" are the focal character and love interest of the transformed heroines of Lescatie in Monster Girl Encyclopedia World Guide I: Fallen Maidens and Monster Girl Encyclopedia Stories: Fallen Brides. With the exception of Druella, the girls' profiles in Fallen Maidens are all written such that their affections converge on the reader's avatar. Fallen Brides short stories gives "You" a mononym, Elt (sometimes shortened to El).
As a child, "You" were a commoner in the Theocracy of Lescatie, a human Order state in the Monster Girl Encyclopedia world. Your parents served the Noscrim aristocratic family, which led to you becoming playmates with their daughter Wilmarina, and even royalty too in the form of Princess Francisca Mistel Lescatie. The young Wilmarina was infatuated with you - a state of affairs much disliked by her father, who seemed to view your playdates as some sort of nefarious plot by your parents to climb up the social ladder. As a consequence, your family was dismissed from the Noscrim's service without a recommendation; in Lescatie, this was tantamount to blacklisting. Your parents died destitute, and you wound up in a church orphanage in the slums run by Sasha Fullmoon's mother.
During your time at the orphanage, you became like a brother to Sasha, and also became friends with another orphan, the half-elf Primera Concerto. Together with Sasha, you convinced Primera to join the Order. You joined too, eventually graduating into the prestigious military unit of the Order of Holy Knights. This brought you under the command of your unit leader Mersé Dascaros, as well as renewing contact with Wilmarina, who officiated at your graduation ceremony. When watching a military magic demonstration you also "befriended" Mimiru Milte, although this mostly involved her swinging by your barracks to throw snark at you at every opportunity.
During Druella's invasion you slept through most of the military panic, "right in the middle of a five storey building, on the left end of the third floor. He was currently residing in one of the private rooms allotted to high-class soldiers". This ended when the newly turned succubus Wilmarina snuck in to rape you. Subsequently Sasha, Mersé, Mimiru, Primera, and Francisca availed themselves of your body as well; and by the time of the Order's counterattack against demon Lescatie, your hours were mostly spent in a perpetual monster heroine orgy:
Despite being a reader avatar for whom vagueness is usually mandatory, there is actually quite a lot of information about "You", in both Fallen Maidens and Fallen Brides. A distinguishing physical feature is bright red hair. Wilmarina also recalls you having low blood pressure since childhood, resulting in being a bit of a sleepy-head. Personality-wise, what brings you to many of the girls' attention is your almost complete disregard for social stratification: treating Wilmarina and Francisca as friends rather than nobles; Mimiru as a kid rather than a magical supersoldier, and Primera as a person rather than a half-elf outcast.