Oh, it took me a long while to find who I wanted. Spectrum seemed like the best match for what I wanted. She was a cop in New Orleans. I figure she's a late-blooming mutant who got driven mad by her powers and was, eventually, found by Dagger.
I've settled on making her a "recruiter" for the school. She goes to find kids that Xavier picks out. Probably helped rescue Julian and his buddies.