Who is Sierva, Sierva Maria de Todos los Angeles, the girl from Of Love and Other Demons and what is their importance?

The twelve-year-old girl is the daughter of the Marquis Ygnacio and his wife Bernarda. Sierva is neglected by her parents and raised by the mistress of the slave quarter. When Sierva is bitten by a rabid dog, there is much uncertainty on what to do. Sierva seems all right, but becomes slightly sick from a fever. She is subjected to a number of healing methods, which are actually various sorts of tortures.