Dean and Crowley

Background
Dean Winchester and Crowley met during the early stages of the Apocalypse, after Becky Rosen told the Winchesters that Crowley was in ownership of the Colt. Much to the Winchester brothers' surprise, Crowley offers the gun to them without a fight. This small alliance gradually develops into a constant on-and-off thing, during which they are sometimes enemies and sometimes allies. Unlike Sam, Dean was more trusting of Crowley when they first met. Crowley also considers Dean a "bestie".

Season 6
The two become enemies following the aversion of the Apocalypse, with Dean threatening to kill Crowley so he would break Bobby's deal. Dean's desire to kill remains even afterwards, though he is stopped by Sam. Crowley further angers Dean by kidnapping Lisa and Ben, so Dean stops hunting for him. Dean doesn't stop however, and continues trying to stop Crowley so he can't open the doorway to Purgatory.

Season 7
When Dean and Sam discover that Crowley's blood is one of the three ingredients needed to make a weapon to defeat the Leviathans, they summon Crowley to tell him what they need. Crowley agrees to help on the grounds that they get the other two ingredients first, in case his blood is used against him. Crowley offers them the location of the Alpha Vampire, whose blood is another ingredient, before leaving.

After Dean and Castiel defeat Dick Roman and get sent to Purgatory, Crowley kidnaps Kevin Tran and leaves Sam all alone, signaling that their alliance has ended.

Season 8
"Crowley. Poncey guy, about yea big, mountain of dicks."

- Dean in Trial and Error Dean returns from Purgatory a year later, and this time he seeks to get rid of Crowley one way or another. This escalates slightly when Crowley begins killing people the brothers once saved. Dean and Sam reunite with Kevin and obtain the Angel Tablet, through the help of Meg who was tortured by Crowley extensively for its whereabouts but refused to give in. Crowley kills her for working with the Winchesters.

When Dean discovers a spell that will lock up Hell forever, he and Sam set out to complete the three necessary trials. Crowley tries to stop them from achieving this goal, and grows noticeably angry when they kill his hellhound. As the third trial requires a demon to be turned into a human, Dean assists in capturing Crowley.

Season 9
After keeping Crowley captive in the bunker for some time, Dean turns to him for help in freeing Sam from Gadreel's control in Road Trip. Dean reluctantly agrees to give Crowley his freedom in exchange and lets him possess Sam to communicate with him. Crowley's efforts allow Sam to expel Gadreel, but while Dean is grateful and allows Crowley to go free, he warns him that he'll kill the demon the next time they meet.

Despite this threat, Dean reluctantly allies himself with Crowley again to take down Abaddon by locating the First Blade. Dean and Crowley hunt the blade together, finally locating the demon Cain who is the creator of the blade. Cain eventually gives Dean the Mark of Cain so he can wield the Blade and helps him and Crowley escape after telling him where to look: the bottom of the ocean. However, while Dean got what he needed, he is furious with Crowley as he realized Crowley used him after seeing him let Dean fight off three demons alone when he could've helped. Crowley confirms this, saying he needed Dean's help to locate Cain and to get him to help as Cain would've never helped Crowley. Disgusted with what Crowley has done, especially as it resulted in the death of hunter Tara, Dean promises to kill him once they've dealt with Abaddon, but sends him after the Blade first as for the time being, he and Crowley still need each other.

During Crowley's search, Dean repeatedly calls him for updates and when they find the First Blade, Crowley releases him from where Cuthbert Sinclair has him bound, allowing him to save Sam. Dean is annoyed when Crowley takes the blade out of mistrust but protects him from a "hunter," not knowing it's really a demon testing him for Crowley. Eventually, Crowley helps Abaddon lure Dean into a trap to save his son, but he secretly warns Dean of the fact, using a code-word Dean had taught him. During the final confrontation with Abaddon, Crowley warns Dean of an attacking demon and seems slightly shocked by Dean's savagery in attacking Abaddon's dead body. Afterwards, as Crowley warned him, Dean lets him go rather than kill him while he's helpless but is angered when Crowley helps his son escape rather than return to his own time.

In the season 9 finale, Dean and Crowley get really close with Dean realizing his powers regarding the First Blade. Crowley also shows compassion and pity for Dean after the latter's death but gives back his blade, bringing Dean back as a demon.

Season 10
After Dean is brought back to life as a demon, Crowley takes him in and the two spend a noticeable amount of time together, with Crowley actively trying to become Dean's friend. Unfortunately, even as a demon, Dean disregards Crowley and this makes him an annoyance to Crowley.

Crowley goes to Castiel and gives him stolen grace, so he can help return Dean back to normal.

Season 11
Crowley and Dean become enemies once again when Crowley decides to take the newly-released and young Amara under his wing. Dean and Sam infiltrate his palace, in an attempt to kill her. When Crowley discovers Dean inside Amara's room, he decides to kill Dean to finally rid himself of the hunter. However, Amara retaliates and attacks Crowley, and orders him to free Dean.

Crowley briefly develops a dislike in Dean over Amara's favoritism in him.

Dean seeks Crowley out again when he finally agrees to help Sam contact Lucifer. They acquire Rowena and the Book of the Damned and make a plan to go the Hell. Crowley later tells Dean that Sam went to Hell without Dean and is now stuck with Lucifer. Dean and Crowley then help each other bind Rowena since she had been working with Lucifer all along, and force her to send Lucifer back to the Cage.

The plan is unsuccessful, and Crowley is dethroned while Dean is left convinced that everything is alright for a couple of weeks. Dean does not see Crowley again until Crowley manages to escape the archangel's clutches, and calls Dean to meet him at a hidden location. Crowley reveals that he possesses the Horn of Joshua and intends to use its power to defeat both Amara and Lucifer. When both Sam and Dean decline to follow through unless they get Castiel back, who is currently acting as Lucifer's vessel, Crowley mentions the numerous bad things Lucifer did to him, but Dean and Sam simply find them amusing.

When Rowena shows up and reveals herself to be "back from the dead", the four of them work together to trap Lucifer and get Castiel to expel him. When initial attempts fail, Crowley enters Castiel's vessel to speak with him directly. Dean and Sam wait outside, insisting the demon hurries up. They soon discover burnt on the forehead of Crowley's vessel a plea for help.

Dean and Sam work together to expel Crowley from Castiel, who admits that he too failed to retrieve the angel. When Lucifer regains consciousness, Crowley decides to bail on the Winchesters for his own safety.

Dean and Crowley do not meet again until weeks later, when God devises a plan to use demons, angels and witches in an attack against Amara. Dean personally goes to meet with Crowley and get him to join the team. When this plan also fails, Crowley remarks that it was a "dog's breakfast".

The two, as well as the rest of the team, return to the Bunker where they drink off as they await the end of the world. At Sam's insistence, the team figure out another way to combat the Darkness, using a Soul Bomb. Crowley remains at the Bunker while Dean and Sam hunt down ghosts.

After Dean sets out on his mission to destroy Amara using himself as the bomb, Crowley and the rest wait at an empty bar. When they notice the sun's light restored to full glow, Crowley is left to believe, along with everyone else, that Dean is dead.

Trivia

 * Jensen Ackles' daughter J.J. coincidentally shares her birthday with Mark Sheppard.

Also Check Out
Sam and Crowley - Crowley's relationship with Sam Winchester.