Board Thread:Questions and Answers/@comment-24098614-20181128000009/@comment-34962159-20190301234408

Index does become important again in the World War III arc. After that she tends to be more of a side character, but she still consistently appears in every (or almost every) novel.

And, Otaku4469, this isn't the first time I've said this to you, but I'll say it again: please, read the light novels. They have the answers to all of the questions you have.