Dean and Castiel

Dean and Castiel have working relationship, as well as a mutual respect for each other. Dean begins to see Castiel as part of the family as their relationship goes on. Castiel respects Dean and helps him whenever he can. Dean gives Castiel the nickname, "Cas".

Background
When Dean was in hell, it was Castiel who pulled Dean out. From that Dean bears the mark of Castiel on his shoulder. When Dean returns he doesn't know it was Castiel who brought him back. But after finding out that it wasn't Sam, Dean looks for further answers and gets help from a physic Pamela. From her Dean finds out Castiel's name. Before meeting physically meeting, Castiel tries to talk to Dean in hopes that Dean might be able to hear his voice. Unfortunately it isn't so and Dean's head hurts when Castiel speaks.

Season 4
"Who are you? Castiel. Yeah, I figured that much, I mean what are you? I'm an Angel of the Lord. Get the hell out of here. There's no such thing. This is your problem, Dean. You have no faith."

- Dean and Castiel

Castiel first physically meet when Dean summons Castiel to a barn where he and Bobby Singer have traps all kinds around the room. The traps have no effect on Castiel. Dean asks Castiel what he is, Castiel answers that he is an angel of the lord.

Dean doesn't believe this, rather doesn't want to. He eventually does though. Castiel comes to Dean after the rising of the witnesses. Dean asks why the angels didn't come to help, and Castiel responds by saying angels have better things to do than "sit" on Dean's shoulder. Castiel tells Dean to show him some respect or Castiel will throw Dean back into hell. This starts Dean on his though process about angels being dicks.

Castiel comes to Dean later on to send Dean to stop something in the past (In The Beginning). So Dean thinks he needs to kill the yellow eyed demon, but can't stop his mother from making the deal. Castiel comes and brings Dean back. Castiel tells Dean to stop Sam and gives him Sam's location.