She is 22 years old, lives in and around the US as a nomad, and often appears as a short-haired athletically built red haired woman.
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Luci was such a lonely and shy child that she only ever had her brother and her imaginary friend, her own shadow, as people she could talk to. At some point in her life, her imaginary friend started to become more real and watch out for her in ways that shouldn't have been possible.
It's very likely that some god or entity took pity on her for being so pathetic that they were like, "holy shit, you need a friend and I got you. As you grow and change, so will your friend."
The only two times this naturally manifested in her at a physical capacity is when she picked up the sniper scope and she was graced with superior sight as well as a tattoo around her right eye.
But as Luci had grown throughout the contract, her imaginary friend gained sentience and become disappointed in Luci's poor performance. So naturally, it jumped at the chance to switch places and show her how it was really done.
Note: The blood shed from Umbra's bullets create beautiful art upon whatever surface lies behind her shot.