She is a demon pony with a mean streak, who is roaming around Equestria, and not allowed back into the underworld! Becuase she kept pulling pranks and being mean towards the other demon ponies there and they kicked her out becuase they were sick of her X)
Now she is stuck in Equestria with the only company she has being an angel pony called Saint Gloria, who she enjoys teasing and annoying X)
She has the ability to create fire and use fire related magic, along with having powers of dark magic too. Since she is not a normal pony, she frequently does a lot of "sinful" things to her body and knows it can not affect her. Lucy is very cheeky and mean, often being the one to try and get ponies to indulge into their bad sides, and she is not above doing some not t so nice acts in order to get what she wants.
Despite this, even she can have her limits some time, as she can realise when she has gone too far with something, and despite her attitude, deep down she is a lonely pony who just wants a friend. Her time in Equestria has softened her up and she's not as bad as she used to be. Even if it is only by a little XD
Although she greatly enjoys being a pain to Gloria, both of them are pretty close and do care for eachother, and have an on-off again relationship.