Someone mentioned that earlier. I am not familiar with My Immortal, but I guess the idea of dark goth girls at Hogwarts is popular. I always like the idea of transforming an existing character, however.
It's a pretty notorious HP fanfic. Among other things, the series is..."reinvented"...so that all the characters she likes are gothic (or "goffik"), and everyone else is a "prep". It's hilariously horrific, a gem of bad writing and possibly the genre launcher. But don't take it as an insult that your Cho looks like one of Gilesbie's reimaginings. It's really a good picture.