Bobby and Castiel

Bobby Singer and Castiel having a working relationship as well as a mutual understanding. They both want to protect Sam and Dean. Although they don't work often together, Bobby and Castiel do protect each other. In fact its not even till Season 5 that they really meet face to face for the first time. Although, Bobby is first to suspect Castiel of betrayal in Season 6. Following events, they are now enemies.

Background
The two first meet when Bobby and Dean summon Castiel to try and figure out who he is. When Castiel arrives, Bobby fires everything he has at Castiel, only to have Castiel keep walking towards him. Bobby is surprised to see Castiel unaffected, especially when Ruby's Knife doesn't work. When Cas gets near Dean Bobby attacks, only to be knocked out by Castiel. Castiel assures Dean there is no harm done. When first approaching the subject of angels, is willing to believe they exist. Later on as Bobby hears the stories about angels, his opinion isn't too high of them. It takes til Season 5 when they meet again.

Season 5
"Are you really gonna bitch to me? Quit pining for the varsity years and load the damn truck."

- Bobby