Ruby

Ruby is a black eyed 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 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 the relationship between the two gradually deepened, the two even becoming sexually active. She 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.

Chapter One
Ruby was somehow brought back, she believes it was Lucifer, but still is not completely sure. In Chapter One, Ruby is introduced in the Grill where she is seated and watching Dean, Sam, Bobby, Castiel, and Crowley conspire with each other about the vampire problem in the town of Mystic Falls. She then leaves shortly after them to go and walk around the town to find where the Winchesters are resided.

As the night fall sets in, Ruby appears to be on a park bench where she is watching four vampires, Damon, Stefan, Elena, and Caroline tak about deciding where to go and what to do with their missing rings. She shares a moment with Damon before getting up and walking away from them to go to the Mystic Falls Motel.

Ruby appears at the motel and watches the Winchesters leave the motel, she then proceeds forcefully into the room, ready to make her presence in the town known. She toys around with Beth by letting her know the vampire who killed her Aunt is still alive more than she could say for her Aunt. Beth then attacks her, causing Ruby to slit her throat, unknowingly that there is vampire blood in her system and tells Bobby she'd be back.

She then walks throughout the town full of Beth's blood, and meets Damon face to face where they continue to speak to each other and create a bond.

Personality
Ruby was aggressive, manipulative, and deceptive. She had a rather black sense of humor; for example, when Sam told her of how Azazel slaughtered eight nuns at St. Mary's Convent in 1972, she quipped, "What's black and white and red all over?" and smirked when Sam chastised her. Ruby had strong religious faith in Lucifer, to the point that she spent two years on an undercover mission dedicated to freeing him which she eventually died on; because of this, she considered herself the best and most loyal of all his followers. She believed that he would "save" all demons. Once she freed him, she was visibly awed and ecstatic. Ruby insisted that she was different from other demons in the sense that she remembered being human although she wished she didn't, and that these feelings compelled her to want to defend humanity; She revealed that at least part of this was a lie but whether or not she did remember some part of her life as a human is unknown. Notably, she did not tell the Winchesters that she had once been human right away. Astaroth implied that Ruby did not want the brothers to know because she was "embarrassed," and Ruby did seem upset afterwards, though she quickly recovered. Once her origins were exposed, she began using her former humanity in her favor to paint herself as a demon who remembered and embraced her past.

Although Ruby sought to earn the Winchesters' trust by helping them and saving their lives if necessary, she did not always present herself as an accommodating ally. She frequently insulted or belittled the brothers, particularly Dean (requiring Sam to mediate between them).

Powers and Weaknesses
Ruby was human, but while she was a human, was also a witch, she knows a few tricks or two besides her demon abilities. Ruby more often relied upon on a combination of trickery, ambushing opponents, martial arts, and usage of the demon killing knife she had than on her supernatural powers. She had black eyes and was noted by Alistair  to be "young" by demonic standards, marking Ruby as most likely an average, low-ranking demon. She was far outmatched by both high-ranking demons and low-ranking angels and was extremely wary of both groups. Nevertheless, she demonstrated herself to be capable in evading and fending off the vengeful forces of Hell.

Teleportation - Ruby often disappeared from or suddenly appeared at one location.

Invulnerability - Ruby was immune to death from conventional means, such as massive blood loss from being tortured though she was barely able to walk afterward due to her injuries. Being shot in the chest, merely irritated her, not even notably hurting her or slowing her down.

Spell Casting - Ruby had immense knowledge of witchcraft and the supernatural, due to being a witch in her human life. Astaroth noted that Ruby had been "one of [her] best," indicating that Ruby had become a powerful witch under her tutelage.

Electrokinesis -  Ruby made the Impala slow down and then completely stop in front of her when she wanted to warn Sam, she's also known to make lights flicker or shut off, to announce her presence.

Possession - Ruby manifested on Earth by possessing humans without the need for their consent. Unlike most demons, she possessed humans without visually or audibly manifesting herself on the physical plane. It also revealed that she could possess a soulless, brain-dead host over an extended period of time without trouble.

Immortality - Ruby could potentially live indefinitely; she was over seven hundred years old when she was killed.

Super Strength - When Ruby fought Sam and Dean, she punched Sam hard enough to knock him against a post and make him fall to the ground, stunned, and kicked Dean to land a short distance away and roll across the floor. She also grabbed Dean and pinned him against a wall, with him unable to get free until she let him go. However, Ruby was usually overpowered by even her fellow black-eyed demons in direct combat and she did not overpower Dean.

Immunity - Ruby was immune to hallowed ground and possibly iron. Ruby entered churches without any difficulty, showing that she was not affected by hallowed ground.

Regeneration -  When in another host, Ruby commented that her brain-dead host was "rotting" without her possessing it, indicating that she was keeping the body from decomposing. She also cut her arms, and let Sam cut her, numerous times to let Sam feed off of her blood, yet her host remained unscarred, which may mean that Ruby was healing her host's wounds.

Super Stamina -  Ruby appeared to not need to sleep, or at least needed less sleep than humans do. Although she did not require food to survive, she enjoyed eating French Fries.

Telekenisis - Ruby had the ability to move and otherwise influence objects with her mind. She used a hand gesture to shut and lock the doors on Dean at the Church while Sam brok the last seal.

Weaknesses

Salt - Most cannot cross salt and feel intense pain when they come in contact with it.

Iron - They cannot cross iron, and can be hurt by it.

Devils Traps - A symbol that can capture a demon if on the wall or cieling, if carved on a bullet and shot in to a demon it will bind the demon into their meat suit.

The Colt - Permanently kills demons.

Ruby's Knife - Permanently kills demons.

Sams Power - Sends demons back to hell.

Death's Sythe -  Kills demons.

Spells - There is a spell that can kill every demon in a one mile radius, requires the heart of a virgin.

Relationships
Sam Winchester - Sam and Ruby were enemies who became lovers and enemies again. Ruby got Sam hooked on drinking demon blood.

Dean Winchester - Dean and Ruby's Ally-Hate Relationship, then turned into an enemy relationship.

Bobby Singer - Allies/Enemies

Lucifer - Ruby worships Lucifer

Damon Salvatore - Possible love interest

Beth Silv - Enemies.

Adam Simons - Allies/ Friends / With Benefits