Not to be confused with Luci.
Lucy was unknowingly turned into a vampire. Sam and Dean encounter her in Fresh Blood.
Background
Lucy is a young woman who Dixon has picked up in a club, and spiked her drink with vampires 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.
Appearance
Trivia
Mercedes McNab is best known for playing Harmony Kendall on Buffy the Vampire Slayer and Angel: The Series where she portrayed a high school girl accidentaly turned into a vampire.