How is Louise what she is in the Red Garden OVA: Dead Girls?
The answer lies within the OVA's title, Dead Girls. The OVA is a homage to the 1992 debut novel of the same name by British science fiction author Richard Calder, which


  tell[s] the tale of a virus that turns pubescent girls into vampiric gynoid dolls called Lilim, and the doomed love affair between 15-year-old Ignatz Zwakh and a Lilim assassin called Primavera.


What's more, it appears that the TV series has also drawn inspiration from the Dead Girls novel, as the Animus-Doral feud and the conditions affecting members of the two warring clans match well with the rivalry between Cartier and Kito and the conditions affecting their dolls:


  years ago during a trade war Cartier infected Kito's fake dolls with an impotence STD and in response Kito sent her own virus to infect the genuine Cartier dolls in Paris - a virus that Morgenstern thinks responsible for the doll plague.


To answer my own question, Louise was probably born a human, but was turned into an android upon contracting a virus similar to that in the novel.