Spoilers ahead for books 2 - 6 of the series.
I just finished reading all six books. Fantastic series. There is something that I don't quite understand, however.
Nadya is an absolute enchantress. Throughout the books, at least since the story with the mirror, it is reiterated again and again that the Twilight cares about a balance between Light and Dark.
Why does the presence of an absolute Light One not trigger a mirror? There is also the grandmother of the Conclave of witches switching from Dark to Light; and of course, Anton becoming a Higher Other until the end of the series.
Why is this? I was convinced throughout the sixth book that Nadya would be important. Okay, she ends up being one of the six, but I think Lukyanenko could have written Svetlana in her stead as well; it doesn't strike me as inevitable.