If it'd be ok, I'd like to use a concept I've had in mind. I'll give a brief synopsis of it just for justification and reference to her abilities.
Her name is Sin, a young woman who came to meet her fate after being murdered while on her way home late one night. While usually that'd mean the end of ones story, for her it was simply a new beginning. Upon her end she was greeted by death itself, or what mortals through time come to refer to as the reaper. It had an offer for her, a deal that if found agreeable would permit her to remain among the living, but it'd come at a price. Death believed that with the arrival of meta-humans came a dramatic decrease in deaths, which left it feeling cheated. Death believed that the natural order was disturbed and if left unchecked would mean an upset in the natural balance. With this in mind, Death offered for this girl to keep her life among the living world but in return she would become an instrument of it's will, to remind mortals that they are not safe from it's embrace and to reintroduce balance by culling meta-humans from the world.
So that's the basic concept. I hope you like the idea! if not I can always rework it so it fits better if need be. Powers and abilities I thought for her to have a relationship with the dark, I don't mean darkness as in the element, but as if the dark was a living breathing entity that came to life in her presence. Her powers and abilities would be naturally themed by it, whether it was a case of moving in and out of shadows, between them or even casting it to swallow people. Her super weakness would generically be light, as it would greatly decrease what she could do. She 'Isn't immortal' she is most definitely able to be killed or captured, killing her would force her to retreat into the dark till she was able to manifest enough strength to return again, capture would be a better alternative. A prison of some kind of light would render her unable to move till the source was removed.
Also this means you'll be less restrained in dealing with me when we come to trading blows 8) Let me know what yo think!