Lucy was bought by a man that marked her, knowingly handicapping her so she wasn't able to use her Dragon form she didn't know about. As well as making it almost impossible for her to protect herself without her master's aid when she truly needed it.
He was not cruel to her, he treated her well, but never let her leave his home, and her prison. She had been with him since she was six in human years.
In Dragon years she was fifteen years old, because time is different in the Dragon world, it is two and a half years faster than the human world, she had learned to fight and transform as well as other things, but when she went to the human world her memories had been erased, in case she had gotten captured she wouldn't give away any information.
When she was born, she was born a dragon, usually dragon children are born as Dragon's as well, but she had been the start of a old and forgotten Dragon species and clan. The said species was named the Twilight Dragon's because when she was born she looked like a night sky.
Before she had gone to earth, she was supposed to be betrothed to the son of the Fire Dragon King. Her mother had sent her to the human world for safety since she was supposed to lead all the Dragons, but they wanted her dead because they couldn't control her, and feared her.
Once she had safely traveled to the human world, she had been caught by some human drug traffickers, and had been put into a auction, sold to the highest bidder.