Crowley

Fergus "Crowley" McLeod was the right-hand man of Lilith and King of the Crossroads Demons. It was revealed by Becky Rosen that Bela Talbot didn't give the Colt to Lilith like she said, but actually gave it to him. Unlike other demons, he is not loyal to Lucifer because he is an angel and realizes that he doesn't care about demons at all and believes that if he destroys humanity then demons will be next. He ultimately wants Lucifer dead so things will go back to normal before the Apocalypse happened. He forms an alliance with the Winchester brothers by giving them the Colt and when it fails to kill Lucifer, he then proceeds to help the brothers get the rings of the four horsemen to put Lucifer back into his cage. After Lucifer was resealed and with Lilith dead, Crowley was made King of Hell. During his reign, his bones were eventually set ablaze by Castiel, reducing Crowley to ash.

Season 5
At the end of "The Real Ghostbusters", Becky Rosen tells Sam that there is a scene in Chuck's books that reveals that Bela Talbot lied to Dean when she told him that she gave the Colt to Lilith. The scene in the book explains that Bela actually gave the Colt to Lilith's right-hand man, Crowley.

Crowley first appears in "Abandon All Hope...", when he appears behind a banker that has summoned a crossroads demon. The man is at first very confused due to the fact his previous negotiations about his deal of getting his bank out bankruptcy were done with a young, attractive woman. Crowley explains to the banker that he is a "big fish" so he did him the honor of coming personally to seal the deal. As Crowley and the banker kiss to seal the deal, they are being spied on by Castiel from a distance. Castiel tails Crowley when he leaves but he is stopped due to Crowley's home being protected by Enochian sigils. Later, the Winchester brothers, with the help of Jo, kill Crowley's two demon guards at the front gate and enter his home, while he's watching television. The Winchester brothers cut off the power to Crowley's home and then confront the demon. Crowley begins to walk toward the brothers, but stops as he realizes that they drew a devil's trap underneath a rug. As he turns over the rug and reveals the devil's trap, two demons sneak up behind Dean and Sam and restrain them. Crowley pulls out the Colt and then shoots the two demons. Crowley explains to the brothers that he wants them to kill Lucifer with the Colt. Dean, who doesn't believe him, asks Crowley why he would want Lucifer dead. Crowley answers that Lucifer is an angel, whose well known for his hatred of humanity, he then asks the brothers if Lucifer has this much hatred for humanity then what are his feelings towards demons. Crowley continues saying that Lucifer thinks of demons as only servants and that once he destroys all of humanity, demons will be next. He then gives Sam the Colt. Sam then asks Crowley about the whereabouts of Lucifer to which he answers that he's in Carthage, Missouri. Sam then thanks him and fires the Colt at his head, only to learn that it has no ammo. As Crowley searches for more ammo, Dean asks him that by helping them isn't he signing his own death warrant and what he'll do if they fail to kill Lucifer. Crowley explains that Lucifer is going to kill all demons anyway and that he'll be going into hiding as soon as he is done helping them. Crowley then throws bullets to the Colt at Dean and teleports away.

Season 6
In the episode "Weekend At Bobby's", Bobby summons Crowley so that he can give him back his soul, as he had given it to him in the previous season in exchange for his help in putting Lucifer back into his cell. Bobby reminds him of this, and asks him to hold up his end of the deal, only to have Crowley refuse. Bobby, having anticipated this, turns off the lights, revealing a devil's trap that he had painted in the floor using glow-in-the-dark paint. Despite being trapped, Crowley reveals that he is being accompanied by a large Hellhound, forcing Bobby to release him.

After capturing and torturing a Crossroad Demon, Bobby learns two things: the first is that Crowley is currently the King of Hell, and Crowley's real name from his life as a human: Fergus McLeod. Bobby then uses Crowley's name and other clues to find out more about his background. With some help from fellow hunter Rufus Turner, he learns that he was born in Scotland in 1661, and he had a son, Gavin, who was the captain of a ship that went down. But, they recovered Gavin's ring, which is on display at a museum in Andover, Massachusetts. With some difficulty, Rufus is able to give the ring to Bobby, who uses it to summon the spirit of Gavin.

He then summons Crowley once more, trapped under a devil's trap drawn on the ceiling. When Bobby notices that he appears slightly disheveled, Crowley tells him that being King of Hell is not as fun as he thought that it would be, seeing that Demons are inherently destructive beings who don't want to listen to him and help him make things better, ironically being able to see why Lucifer hated them so much. After refusing once again to return Bobby's soul, Bobby reveals Gavin. But, after a faux display of affection, he reveals that he cared nothing for his son. Bobby says that he knows that, but says that he and Gavin had a long talk and that Gavin was willing to tell him all about Crowley. It's revealed that in life, Crowley was a tailor who made a deal with a Crossroad Demon to get "an extra three inches below the belt". He also reveals that Gavin told him where Crowley's mortal remains are buried, and hands him a phone. On the other end is Dean, who is with Sam in Scotland, having flown there to dig up Crowley's bones and torch them if he doesn't give Bobby back his soul.

Realizing he has been tricked, he gives back Bobby his soul -- leaving in the part of contract that allows Bobby to walk-- and after he is released, he teleports to Scotland to retrieve his bones.

In Caged Heat, the facility where Crowley has been hiding and torturing the Alpha creatures is discovered by the Winchesters through Samuel. Making a deal with the demon Meg, who has a grudge against Crowley, the boys and demon infiltrate the facility, only to be double-crossed by Samuel. Samuel then reveals that Crowley plans to resurrect Mary Winchester in exchange for his help. After escaping their prison and freeing Meg from torture, the trio trick Crowley into a Devil's Trap, leaving him at their mercy. Keeping their deal, Meg tortures him for the boys, trying to force him to retrieve Sam's soul. In the midst of their conversation, Crowley gets Meg to step in the circle accidentally and breaks free allowing him to throw the Winchester brothers against wall. As Crowley gloats over his apparent victory, Castiel reappears with a bag containing Crowley's bones. After asking one more time about Sam's soul being returned, Crowley is deemed unhelpful and useless. Castiel then promptly sets the bones ablaze, leaving Crowley nothing but a pile of ash.

Relationships

 * Lucifer (Father, Creator)
 * Demons (Siblings, Subordinates)
 * Lilith (Possible Lover, Superior, Demonic Sister)
 * Gavin MacLeod (Son)

Season 5

 * "Abandon All Hope..."
 * "The Devil You Know"
 * "Two Minutes to Midnight"

Season 6

 * "Weekend At Bobby's"
 * "Family Matters"
 * "All Dogs Go to Heaven"
 * "Caged Heat"

Trivia

 * Crowley can be considered the main antagonist of the first half of the sixth season.
 * It is possible that Crowley may have the fourth wall awareness since he remarked to Castiel in "Caged Heat" that he hasn't seen him all season.