Not to be confused with Luci.
Lucy was a woman unknowingly turned into a vampire.
Lucy is a young woman whom Dixon has picked up in a club, and spiked her drink with vampire's blood. He takes her back to his place, but she escapes and starts killing people to feed. When Sam and Dean capture her, they find that she is unaware of what's happened - she thinks she is on a drug high and can't come down. Seeing no alternative, and with Sam's agreement, Dean kills her.