Idiosyncrasy. Ana has to save from extinction a rare species: a vampire. It is a necessity created by his progressive ideas. Environmentalism and conservationism against the ignorance and brutality of the hunters, the exploiters.
Maternal protective instinct. The vampire is a helpless being and Ana assumes his protection. Like the wolf suckled Romulus and Remus, Ana feels the need to breastfeed (symbolically) the lost wolf and protect against their murderers.
Love. Ana is passionately in love with the vampire. This is another need that Ana had. In full crisis of 30, when she missed to have lived a true love, deep, she meets a being that fits her innermost, aesthetic, ideological tastes, that fills other needs such as maternal and protection, ultimately the elements that generate the passionate love in youth.
Hidden desires. The vampire embodies some symbols and occult beliefs. It is the devil who love witches. Represents a romantic and gothic, cultured and elegant European aristocracy. Ana observes an intellectual and moral superiority manifested in relation to the people around her. It borns in her an atavistic sense of cruelty and contempt for the lower, the uncouth, so much so that leads her to murder.