In the Beginning
Cas lies in front of him, collapsed on the floor and drenched in Jimmy's blood. Dean shouts his name, gripping the angel's arm as he tries to wake him. Bobby says he's gone, that it's too late. Dean shakes his head. He can't believe it. He won't. Cas can't be gone, he can't go. Dean needs him. Dean needs Cas as much as he needs Sam.
He stands and tries to hold back the tears swimming in his vision. He warned him, dammit. He told Cas it was a bad idea. To play God. And now look at him. Dead. Dean had lost his angel. His friend. There had been no one like Cas. No one. And there never would be again.
Suddenly Cas gasps and his eyes spring open. Dean drops to the floor, speechless. He and Bobby bring the angel to his feet. Dean is clinging to Cas, relieved and bewildered. His angel is okay. His angel is not dead. His angel is alive.
But then Cas twitches, and his face contorts. Dread shoots through Dean like his blood has been set on fire. He can't bear to lose Cas again. He just can't.
"Run!" Castiel screams, but it's too late. The Leviathan take control, and Cas is gone. The Leviathans' victory words fall short in Dean's ears. All he can think about is Cas. His angel. His friend. Gone. And when the Leviathan leave, he would have lost his mind if Bobby hadn't dragged him out to look for Sam.
Later, when Bobby asks him if he's okay, Dean lies. He knows Bobby sees right through him, but it doesn't matter. He had to be strong for Sam. For Bobby. And most of all, for Castiel. He has to keep his head. Or his angel will have died for nothing. Dean has to kill those Leviathan bastards. For Cas. For his friend. For his angel.
And after Dean wakes up from the nightmare were he watches Cas die all over again, he realizes something.
He's been referring to Castiel as his angel.