I'm still working on her backstory but here's what I've got so far:
When she was younger, she was akumatized but the akuma was in her heart so the two miraculous holders of her time couldn't really defeat her easily. The weapon of one of the miraculous holders was magical bow and arrows so she shot her in the heart, accidentally fusing her with the akuma. She can transform into her akumatized form at will but still had her free will. The thing is that she is attaked by both miraculous holders and akumas. She moved to America after the incident but moved back in douring high school. When she returned to Paris, she started interfeering with akuma attacks on the sidelines, never revealing herself to the miraculous holders.
That's all I've got so far but I would appreciate hearing what you think!