Ruby

Ruby is a demon who frequently fought on the side of the Winchesters. She formed an alliance with an uneasy Sam by promising to help him get Dean out of his deal at first, but the relationship between the two gradually deepened beyond friendship.

Season 3
Ruby first appeared in the season three premiere, stalking the Winchesters during their hunt. She appeared later during the episode's climax, saving Sam by killing Pride and two of the other sins with a knife that can kill demons and vanishing soon after.

In "The Kids Are Alright", Ruby tracked Sam down at a local diner. He asks her about the knife and she claims that she is a hunter. She reveals her knowledge of the Yellow Eyed Demon's special children and that Sam is the sole survivor. Before disappearing again, she told Sam to look into his mother's friends. At the end of the episode, she returned to their motel room to meet with Sam. Sam told her that they were being hunted down, one by one, and she claimed that it was the doing of the Yellow Eyed Demon. Sam finally confronted her about how she knew so much, and Ruby revealed to him her true nature by flashing her black eyes. Sam wanted to banish her, but she baited him, promising to help him break Dean's deal.

Ruby appeared again in "Sin City", this time to Bobby while he was testing the new Colt. He shot her with it, and when it failed to kill her, she offered her help rebuilding it. Whatever she did worked, as the newly constructed weapon was able to kill Casey and Father Gil.

In "Malleus Maleficarum", Ruby appeared again to help the Winchesters get to the bottom of what appeared to be a coven of witches. After several of the members die, Ruby appeared after the demon possessing Tammi revealed itself. The demon claimed that Ruby used to be "one of her girls" when she was alive, revealing that she was once human. At the end, she had a conversation with Dean outside the motel room. She told him that all demons were once human, turned into monsters during their time in hell. She compared it to the movie Hellraiser, and revealed to Dean that ultimately, there was no way to save him from his eventual fate.

In Jus In Bello, Ruby fought her way into the police station where special agent Victor Henriksen was keeping the Winchesters while it is being besieged by a demonic army. She offered a way to get rid of the demons: by sacrificing the virgin, her spell would kill all demons within it's range, including Ruby. The Winchesters declined her offer and let her out. After escaping the demon army by using a mass exorcism, the brothers saw on the television that Lilith destroyed the station, killing everyone inside it. They were met by a very angry Ruby, who informed them that their plan "was the one with the body count". She gave them hex bags to "throw Lilith off [their] trail" and left.

She appeared again in the season finale, helping the boys track down Lilith. However, they tricked her and stole her knife, as, after Bela stole the Colt, they needed a way to kill Lilith. She appeared to have escaped, as she showed up later, demanding the return of her knife. She helped the boys infiltrate the house Lilith was supposed to have been hiding in, but they discovered that Lilith had changed hosts. Lilith unleashed her hellhounds against Dean, and Ruby and the boys holed up in a room protected by goofer dust. Ruby asked for the knife, apparently hoping to fend Lilith off, but Dean suddenly reveals that the woman in the room with them isn't Ruby: Lilith had taken over her old host. Her con revealed, Lilith in Ruby's old body let broke the circle and let the hellhounds into the room, and they killed Dean while Sam watched. She then tried to kill Sam with the same white light she used to kill Henriksen, but the light had no effect on him. A startled Lilith left Ruby's former host, leaving Sam surrounded by two corpses.

Season 4
After Lilith possessed her original body during the end of Season 3, Ruby's whereabouts were unknown for a while. Her new body first showed up in Sam's room when Dean revealed he was alive. Sam pretended she was a random hookup and she left. He met her later in a diner, and she agreed to help him find out who pulled Dean out of Hell and why. Dean, however, is unaware that she survived. In Are You There God? It's Me, Dean Winchester, Ruby appeared again to Sam, asking him if the rumors were true about an angel saving Dean. Not wanting to get mixed up with angels for obvious reasons, Ruby left. However, this decision was less than permanent, as she appeared in the next episode, picking up Sam, helping him accomplish some yet unknown goal.

In "Metamorphosis", Ruby and Sam interrogated a demon on the whereabouts of Lilith. When he wouldn't talk, Sam used his psychic powers to exorcise him. Ruby asked him if he stopped getting headaches, and replied that his powers were growing. At that moment, Dean entered the room. Upon discovering Ruby was alive, he attacked her. However, she eventually got the upper hand before Sam told her to back off, which she did.

In "I Know What You Did Last Summer", Ruby dropped the Winchesters a tip about a girl Anna Milton, who is wanted by demons. This prompted Dean to ask Sam why he trusts her so much. Through the use of flashbacks, we learn that Ruby returned after promising Lilith that she would kill Sam. She betrayed Lilith and assisted Sam once again. Sam later scorned her for using a living body. Ruby left, and went to possess a brain-dead coma patient. Using this body, she began to instruct Sam in the use of his psychic powers. She seduced him, and helped him off his path of self-destruction. Back in the present, Ruby contacted the boys using a maid's body, giving them an address. They head to the cabin, and discover that Ruby has rescued Anna Milton from the mental institution. However, Castiel and Uriel soon show up, declaring that the girl has to die.

In "Lucifer Rising", Ruby and Sam make for Lilith to kill her before she breaks the Final Seal. In doing so, Ruby kidnaps a young nurse possessed by a demon so that Sam can bleed her and drink the blood to increase his power. The demon temporarily releases her control and lets the nurse out, making Sam uneasy about killing her. She accompanies Sam to the church built on the ground where Lucifer was caged and witnesses him kill Lilith. However, there was a catch: Ruby reveals that she was working for Lilith all along, and tells Sam about the breaking of the Final Seal. She is killed by the timely arrival of Dean Winchester with an assist from Sam shortly after her treachery is made known.

Trivia

 * Apparently, Ruby used to be a human witch "back when the plague was big".
 * Genevieve Cortese, who played Ruby in Season 4, is married to Jared Padalecki (Sam) in real life

Powers
Ruby demonstrates several powers common to demons, such as possession and telekinesis, and a vast knowledge of spells and magical rituals (Hex Bags) due to her being a witch when she was alive. However, as she was apparently Lilith's closest confidant, it can be inferred that her powers may be much greater, though she is still just a black eyed demon.

Season 3

 * "The Magnificent Seven"
 * "The Kids Are Alright"
 * "Sin City"
 * "Malleus Maleficarum"
 * "Jus In Bello"
 * "No Rest for the Wicked"

Season 4

 * "Lazarus Rising"
 * "Are You There God? It's Me, Dean Winchester"
 * "In The Beginning"
 * "Metamorphosis"
 * "I Know What You Did Last Summer"
 * "Heaven and Hell"
 * "Criss Angel Is a Douchebag"
 * "Sex and Violence" (voice)
 * "On the Head of a Pin"
 * "When the Levee Breaks"
 * "Lucifer Rising"