Ruby

Background

She debuts in the third season premiere "The Magnificent Seven", where she trails Sam and rescues him from a group of demons, killing them with her own demon-killing knife. In "The Kids Are Alright" she reveals to Sam that she is a demon but claims to be different from the others, and promises to save Dean from the deal he made to bring Sam back to life. In "Sin City" she restores power to the Colt, and "Malleus Maleficarum" reveals that she was a witch during the Plague who sold her soul to a demon.

In the third season finale "No Rest for the Wicked" she offers to train Sam to use his demonic abilities to kill Lilith, the demon who holds the contract for Dean's soul. Dean tricks her into a devil's trap and leaves with Sam to face Lilith, and Lilith expels her from her host body. She returns in the fourth season in a body recently declared dead, now played by Cortese, and Sam comes to trust her completely.

Ruby trains Sam to exorcise and later kill demons, and "On the Head of a Pin" reveals that she is feeding him her demonic blood to boost his powers; by "The Rapture" he has become addicted to it. In the fourth season finale "Lucifer Rising" she keeps Dean from interfering while Sam kills Lilith, then reveals that she is a double agent working for Lilith who has tricked Sam into setting Lucifer free with Lilith's death. With Sam's help, Dean kills her with her own knife.

In the fifteenth season episode "Destiny's Child", Castiel goes into the Empty, the afterlife of angels and demons, to ask Ruby where she hid the Occultum, and she whispers its location in exchange for his help in getting out of the Empty. Series creator Eric Kripke cited budgetary reasons for Cassidy's departure after the third season, and Cortese was hired to recast the role for the fourth.