[quote=@leyroyjenkins]
What you suggest to make her unique?
[/quote]

She doesn't need to be a killing machine or have a particularly tragic backstory. In fact, if she's already super-duper skilled,  it would make her less interesting to play with because there's no room to grow. She just needs to be someone with a story that's interesting to watch.

Maybe she could be a twin or sibling to someone who seems to always get recognition and praise, while she's usually ignored. That way, her joining a gang makes more sense since she's trying to rebel, and she could be motivated by her desire not to be outdone by her sibling.