Ruby is the Love Interest of Sam Winchester from the CW TV show Supernatural. She was played by Katie Cassidy and Genevieve Cortese, who later married Jared Padalecki, who played Sam Winchester.
Ruby was a witch when she was human and she lived during the Black Death when the Horseman Pestilence was eradicating humanity. Ruby decided to sell her soul to Satan and became a witch for powers. She was damned to Hell due to this and the demon Astaroth who was her master took her to Hell and raised her as a demon.
Unlike many demons, Ruby went for martial arts instead of magical powers. She was also more human than many other demons, having doubts and a sense of honour.
Ruby first appears when Hell is opened and many demons escape in a church in Wyoming. Ruby then possesses a blonde woman and she stalks Sam. At one point, she first meets Sam by saving him from the Seven Deadly Sins, killing two of them and saying "I'm the girl who just saved your ass" when Sam asks who she is.
Afterwards, they get to know each other. Ruby says she's a demon, scaring Sam, but he decides not to kill her because she could be useful. This angers Dean Winchester but he eventually has a talk with Ruby about Hell, because Dean himself is damned to Hell because of a Faustian deal he did to save Sam. Ruby says Hell is awful even for demons and Dean will have a horrible time there.
Ruby breaks into the house where Lilith had been staying, accompanied by Sam and Dean, but Lilith possesses Ruby and kills Dean. Then Sam tries to kill Ruby but she flees her host, leaving her host presumably dead.
Ruby is the hidden main antagonist in season 4 because of her manipulation of Sam and desire to free Lucifer and destroy humanity.
During the three months Dean spends in Hell (or thirty years in Hell time) Ruby's relationship with Sam goes openly romantic, with Ruby getting naked to seduce Sam and making him drink her blood. Although Sam sees it as wrong initially, he gives in. Sam sees the relationship as romantic but it is really just a perversive one. Sam is always drinking Ruby's blood to boost his supernatural powers.
When Dean returns from Hell, having been freed by the angel Castiel, Ruby knows angels are after them so she flees. When she returns, she continues her relationship with Sam. In fact, Ruby is in a T-Shirt and knickers when Dean returns from Hell. Ruby acts like she doesn't know Dean but then Dean realizes that it's Ruby. Sam had berated her for possessing a live human so she possessed a comatose brain-dead woman instead. Ruby goes between kissing Sam and making out with him to fighting renegade demons and trying to help Lilith free Lucifer. Ruby tells Sam that Lilith is the last of the 66 Seals imprisoning Lucifer and that killing her will keep the Devil imprisoned, and when the angel Zachariah tells Dean the opposite, that killing Lilith frees Lucifer, because Lilith is the final Seal, Dean runs to the church where Sam and Ruby are, in time to see Ruby smirk at Dean and slam the door on him. Then Ruby begs Sam to kill Lilith. He does so, and then her blood forms a door on the floor. Light begins to rise and Lucifer is coming. Ruby says how Sam followed her commands, so it's not really her fault, she was the best villain out of all of them. But then Sam grips her and Dean runs in and uses Ruby's own Knife to kill her.