Chouko (Loveless) -- Who is this character?

Chouko dies before the series begins, but I've always wanted a fic about her and she is indeed a canonical character in Loveless. I don't have a quick way of finding the exact pages, but Ritsu and Soubi discuss her at her graveside, we see her body, and and it's noted at least once that she rejected Ritsu and left the school he leads and he had, um, issues over it.

Agatsuma is the surname of her son. Her surname is uncertain.