Azazel a.k.a The Yellow-Eyed Demon

Azazel has children of his own, notably his daughter, the demon who possessed Meg Masters, an antagonist in the first season who was exorcised and sent back to Hell by Sam Winchester. These "children" are demons that he loves very much as he asks Dean why he thinks it's permissible for humans to have families but not for demons, and he tortures Dean for killing one of his "children" who was beating Sam bloody in a back alley after the Winchester brothers save their father in Devil's Trap. The only thing he seems to fear is the Colt, a special gun which can kill almost anything. He spent the first season trying to obtain this weapon. At the beginning of the second season, he procures it from John Winchester, along with John's life and soul, in exchange for saving the life of John's son, Dean.

Sam and Dean think, for a time, that Azazel's attacks follow a set pattern. The attack would occur six monthes after the birth of a "gifted" child, would originate in the child's nursery, and the mother of the child would die by being pinned to the ceiling, bleeding from the stomach, where she would burst into flames. These deaths were result of him fulfilling deals he made with one of their parents ten years prior. Azazel stated he simply needed permission to do something in their home and, as long as he was not interrupted, no one would be hurt. In exchange they would receive something from him.

This was the case with Sam and his mother Mary Winchester, and with Max Miller and his mother. Mary had made the deal ten years prior to save John's life and end her career as a Hunter. The demon would tell Dean in the episode "In the Beginning" she was his favorite. However, the pattern is broken when Sam's girlfriend Jessica is killed (Azazel claimed she was holding Sam back from honing his skills), and in "Simon Said" with the case of Andy Gallagher and his twin brother Ansem Weems, who were both adopted. Andy's adoptive mother was killed, but Ansem's was not.

The full scope of his powers is unknown. He mostly appears by possessing the bodies of others, and is only identifiable when he presents yellow eyes. He seems to possess telekinetic and pyrokinetic powers, which he uses elaborately to kill his victims. When not in possession of a body, he can exist as a black smoke, and at one point manages to dodge a bullet from Sam by teleporting. He holds considerable influence and power, at one point he was even able to posses a reaper and save Dean's life. Unlike other demons on the show, he appears to be immune to holy water, and most other forms of defense against demons.

In "All Hell Breaks Loose, Part 1", Azazel appears to Sam in a dream. In this twisted dream, the demon informs Sam that he is in a contest against all the other "special children" like him. Azazel then notes that Sam is his favorite, so he shows Sam a vision of the night that he visited him. It is shown that Azazel bleeds into the child's mouth, as sort of a ritual. Azazel also shows that Sam's mother, Mary, recognizes him and was only killed because she was at the "wrong place at the wrong time". Series creator Eric Kripke stated that her relation to Azazel was supposed to be addressed in the third season, but was pushed back to the fourth season due to the writer's strike.

Azazel's last appearance chronologically is in "All Hell Breaks Loose, Part 2", where he approaches Jake in a dream to tell him what to do now that he is the last survivor of the competition. He threatens Jake's family, and the soldier listens. When Jake is asked by Azazel to do something for him, Jake is given the Colt and immediately points it at him. Feigning fear, Azazel shrugs off the Colt pointed at him and says that Jake would rather have health and wealth than to shoot him. He promises security for his family as well, causing Jake to lower the weapon. With Azazel's promises, Jake is coerced into entering the Devil's Trap and into using the Colt as a key to open a gate to hell. The rush of demons from the gate breaks open a Devil's Trap, allowing Azazel to enter the area. He then appears behind Dean, who attempts to shoot him with the Colt. However, Azazel telekinetically grabs it from him and forcefully throws Dean onto the ground, telling him how thankful he is to Dean that he revived Sam since he liked him better than Jake. He also informs Dean that whatever Dean brought back is possibly not "100% pure Sammy". He then aims the Colt at Dean and prepares to kill him, but the spirit of John, who escaped through the gate, appears and grabs him from behind, pulling him out of his human host. Azazel escapes from John's grasp and returns to his body, but Dean already has the Colt by then and shoots him in the heart, killing him. Dean then triumphantly stands over the demon's "body", recognizing his and Sam's victory over the thing which has given their family so much death and heartache. However, with the release of hundreds of demons, the Winchester brothers accept the fact that they have more work to do and more demons to hunt down and kill, some possibly worse than Azazel.

Azazel's ultimate goal, what he called his "end game" was something more than just unleashing a demon army and finding one of his children to lead it. He, however, had managed to cover his tracks well enough to leave even angels guessing at the nature of that end game.

His "end game" was revealed in Season's 4 'Lucifer's Rising'. Azazel found the location of the final seal to free Lucifer, which a convent was later built on. He possesses a priest and murders the nuns in the convent as a sacrifice. Lucifer speaks to him through the body of a nun and tells him how to set him free: That he must first free Lillith, whom at this point is locked away in hell (this is implied to be the reason why Azazel had Jake open the gate to hell), and then find a special child to kill Lilith for the final seal to be broken. The main goal all along was for him to use Sam to open the final seal. In this episode he refers to Lucifer as his "father", however, this is shown to be done by other demons, because Lucifer is their "creator".