"Plague"

Plague is a very strange girl. Clearly retarded and seriously ill, she only speaks with the simplest words with a strange tone, modified by the gas mask she always wear. She stinks "like she just ate a cemetery and puked it out right after", according to Valentine. She is constantly followed by a legion of flies and refuses to wash herself, no matter how you put it.

Everybody hates her, but in a very strange way. Everybody would sincerely find her cute and nice if she wasn't soo retarded and dirty... and everybody knows it.

Which basically means that the few people who protect her also hate her for being too stupid to realize the danger they saved her from. She is a very passive person, never getting involved in anything, just following people or standing somewhere. She seems too physically and mentally lost to feel any sadness about it, and her stench keeps the curious away.

In fact, plague is the only descendant of a family who devoted itself to sacrifice Ish, again and again, since the Aztec times. Which basically means that she's the only human in the world who knows how to give Ish his memory back. And with it, the way to translate the Voynich and discover how to destroy the Leto. The problem is that her family took its role so seriously that they couldn't bear anybody from outside the family to do it. Seven centuries of random incest made Plague the stupid, dirty but cute and useful little girl she is.