Castiel and Crowley

Background
Castiel and Crowley are from two separate, rivaling species and do not meet until the events of the Apocalypse in season 5. Their relationship is mostly due to their mutual connection to the Winchester brothers, by whom they are often called in for assistance.

Season 5
Castiel is unaware of Crowley's existence until the latter arrives at Bobby Singer's house in Two Minutes to Midnight, revealing that he made a deal with Bobby in exchange for the location of the Horseman Death. The two do not speak to each other, but remain aware of one another all throughout Crowley's participant in stopping the Apocalypse.

Season 6
After returning to Heaven, Castiel found himself in conflict with Raphael. Unable to fight back, he nearly sought the help of Dean, but was reluctant since he believed Dean needed peace for once. Crowley took this opportunity to offer his own help, by promising Castiel an abundance of souls from Hell. Castiel used these souls to declare a civil war against Raphael.

Later on, Crowley and Castiel began working to open up Purgatory for even more souls. This alliance was to be kept a secret, and when the Winchesters became involved, Castiel pretended to fight against Crowley. The most prominent example was pretending to have burnt Crowley's bones, leading everyone to kill Crowley was dead.

However, Eve was well-aware of Crowley being alive, and told the Winchesters this. This led Sam and Dean to begin suspecting Castiel. Although Castiel did his best to hide his tracks by killing demons such as Ellsworth, a single slip-up drove the brothers and Bobby to trap him in a ring of holy fire and interrogate him.

Crowley notes that Castiel had been keeping this a secret so the brothers would see him as noble, and by doing so, Castiel could convince himself that he indeed was. Crowley was also becoming unimpressed with Castiel's inability to betray the Winchesters, and blames Eve's death on Castiel.

Castiel himself had never intended to remain loyal to Crowley, and simply wanted to restore Heaven. He proved this by turning against Crowley and forcing the demon to join sides with Raphael, only for Castiel to have tricked them and already obtained the souls from Purgatory.

Season 7
Crowley knew that the soul-empowered Castiel wanted him dead, but when approached by the angel, he was offered a chance to remain alive since Castiel wanted to keep Hell in order to keep Michael and Lucifer trapped in their prison.

Castiel's tenure as the new god was cut short by the Leviathans he had unwittingly consumed, leading to his death and the unleash of the beasts upon the Earth. Crowley attempted to work with the Leviathans, but their leader, Dick Roman, did not wish this.

This drove Crowley to side with the Winchesters again, by offering his blood to fuel their weapon. It was during this encounter that Crowley met the newly-resurrected Castiel. He clearly expressed his desire to kill the angel, but chose not to since Castiel had lost his mind, to the point where he offered Crowley honey.

Castiel himself would ultimately be sent to Purgatory, along with Dean, allowing Crowley to kidnap Kevin and Meg and leave Sam all alone.

Season 8
Upon Castiel's return from Purgatory, he was under the command of Naomi, who wanted him to obtain the angel tablet. Crowley also sought after said artifact, and went as far as to torture the angel Samandriel for its location.

Castiel attempted to rescue him with Sam and Dean's help, but failed to do so before Samandriel blurted out the information Crowley needed. Castiel continued to hunt down the tablet, eventually finding out that Meg was in Crowley's custody and had not given up the tablet's location.

Castiel and the Winchesters, along with Meg, traveled to Lucifer's Crypts to obtain the artifact. Crowley arrived shortly after, but he and his demons were stalled long enough for Castiel to run away with it.

Nevertheless, Crowley hunted Castiel down and ultimately achieved in removing the tablet from Castiel's stomach. A disappointed Castiel then began working to do the Heaven's trials, while Dean and Sam did the Hell trials, much to Crowley's chagrin.

Season 9
Crowley was subsequently held captive by the Winchesters to use for the third trial, and despite them forfeiting the trials, he remained their prisoner. At Castiel's insistence, Crowley was asked to help with getting rid of the angel Gadreel, who had been given permission to possess Sam in order to heal his injuries.

The betrayal of Gadreel and his own frustration led Dean to side with Crowley, something both Sam and Castiel were not happy about. For the remainder of the season, Crowley and Castiel did not interact.

Season 10
When Dean, as a demon, became a problem for Crowley, the King sought Castiel's help in taming Dean. This was achieved by rescuing Castiel from the angel Adina, and providing Castiel with Adina's grace.

They continued to help each other when the Mark itself proved problematic. Crowley captured his mother and forced her to remove the Mark, while Castiel mostly stood by. However, upon completing the spell, Rowena cast the Attack Dog spell on Castiel, in order to kill Crowley.

The demon managed to survive the encounter, however.

Season 11
Castiel's recovery from the spell forced him to stay indoors for a majority of the beginning of the season. It wasn't until Castiel met with the Darkness and was carved up with a message, that he met with Crowley again.

Castiel arrived in Hell with the Darkness' message, just as Dean, Rowena and Crowley were preparing the spell meant to send Lucifer back to his Cage. Upon hearing Sam was in danger, Castiel joined Dean in confronting the archangel, despite Crowley's protests.

As with the Winchesters, Crowley was unaware of Castiel's ultimate decision to become Lucifer's vessel. After the Winchesters left, Crowley was approached by what he believed to be Castiel, until the angel prompted him to "guess again".

Crowley spent weeks being tortured by Lucifer in the guise of Castiel. This is not affect nor change his opinion of the angel, but when he finally escaped and got his hands on a Hand of God, he was willing to kill Lucifer even if it meant killing Castiel as well.

It was at Sam and Dean's insistence that Crowley agreed to expel Lucifer first. When the angel suppressing sigil failed, Crowley was forced to enter Castiel's vessel to speak with him directly.

Inside Castiel's mind, Crowley discovered the angel to be under a sort of trance, uncaring about the outside world. Crowley deduced that this was Lucifer's doing, and urged Castiel to "snap out of it". He was unable to convince the angel to do anything, and was soon approached by Lucifer. In the ensuing attack, Castiel seemed to only care for his television.

Crowley meets the real Castiel again in Alpha and Omega, after Lucifer had been expelled. The two do not interact, but stay within each other's presence until the conclusion of the final battle.