After knowing about Nigredo, the elder sister of Albedo and another younger sister of Albedo, I was quite amused by the fact that, none of them appeared in Manga and Anime. Why?
By the wiki, Nigredo is less evil but her face is horrible and she is a reasonable person. Than why is she kept in the prison and what wrong she has done to be put in a prison.
Probably bc the creators of both the Manga and Anime are unsure where the author is heading with Nigredo. Nigredo and Pestonya were involved in freeing some humans that were taken when Demiurge attacked Re-Estize. But that isn't explained until Vol 10 and Side Stories. So far she has a very minor role that the creators may have deemed insignificant. Plus the have a limited number of panels for Nigredo in the Manga and same also goes for the Anime.