Ruby was a demon devoted to Lucifer 's return. After Azazel 's death, she conspired with Lilith as to how they would free Lucifer from his cage : while Lilith broke the seals , Ruby protected the remaining " special child " Sam Winchester and prepared him to break the final seal and become the host to Lucifer. This plan was known only to herself and Lilith , causing Ruby to be regarded as a traitor by Hell. At first, Ruby formed an alliance with an uneasy Sam by promising to help him get his brother out of his crossroads deal, but once this was revealed to be false, offered to train Sam to be able to kill Lilith in revenge for Dean being sent to Hell.
During Dean's sojourn in Hell, Ruby tightened her hold by being useful and 'helping' Sam, saying what he wanted to hear, addicting him to demon blood, and being there through his grief to the point that the relationship between the two gradually deepened and became sexual. She molded him into the perfect vessel for Lucifer and manipulated him into breaking the final seal by killing Lilith , but once her deceit was realized by the brothers, she was killed with her own knife.
After learning that Ruby had the Occultum before her death, Castiel visits the demon in The Empty , awakening her in the process. In exchange for the location, Ruby requests that Castiel get her out of the Empty which would resurrect her. Castiel agrees to her request to at least try to free Ruby.
Ruby began life as a human who was alive during the peak of the Black Death, which took place from to in Europe that killed nearly million people. She might have lost loved ones during the plague, as she comforted Sam in a flashback in " I Know What You Did Last Summer " by saying that she knew how it felt to lose someone, though this may have just been manipulation.
Ruby became a witch by selling her soul to Astaroth. Their interaction in Malleus Maleficarum hints that they were in a sexual relationship with one another. As a result of her deal, Ruby's soul went to Hell after she died, where she was tortured to the point that she became a demon herself. During this time, Ruby worked with Anael on occasion who was one of the few aware of her true allegiance.
