Even I don't know how her hair became such an identifiable and recognizable part of Riley's overall design. You can see even in her earliest appearance in Chapter 5 how I hadn't quite settled on what her hair was going to look like yet, but then, there was a lot about her I still hadn't quite figured out by that stage in the story, and her hair wasn't the highest of priorities.

In any case, this is one possible in-universe explanation for how it got that way.

https://www.godslayerscomic.com