CONTAINS MAJOR SPIOLERS! DO NOT READ UNLESS YOU'VE SEEN EPISODE 22! Yeah, so Destiel (Dean Winchester/Castiel) , (which means slash so don't read this unless you're okay with that), although Dean is only mentioned in this.
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After Gadreel had recovered from almost dying at Dean's hands, he spoke with Sam and Castiel. He'd used Sam as a vessel for a long time, so he knew that he and Castiel were trustworthy. It had confused him why Dean had been so different, so much more aggressive than before, at first. He'd been less willing to trust the others- before Castiel told him about the mark of Cain on Dean's arm. It made sense now. He refused to be anywhere near Dean.

He told them that he would speak with Castiel. Tell him everything. He still believed that Metatron had to be stopped. Angels were dying. People were dying.

So he answered Castiel's questions truthfully and as best as he could. Until he could add no more knowledge of use.

"Castiel, may I ask?", he said as Castiel got up to leave. Castiel nodded.

"You love him; the human."

Castiel squinted at him. "Dean?"

"Yes."

"Yes." Castiel replied. "Your question?"

"Does he know?"

"I don't know."

"Does he love you?"

"I don't know. Do these things matter?"

"Not to us, no. But humans love differently. I have observed this in them. They are not always content with just companionship as angels are. To them friendship and romantic love are very different, expressed, differently."

"Would you advise me to speak with him of this?"

"It is not my place."

"No. It is not."

At that point Sam came back in."Hi Cas, everything okay?"

"Yes. Everything is fine. How is Dean?"

"I think you need to speak to him Cas. He's-" Sam gestured vaguely. Humans could be so ambiguous. So confusing. Castiel, however, seemed to get the message.

Sam looked at him, as Castiel left the room. " Uh... I guess you can just stay here...? I mean you can't really to back to Metatron with that," he said, gesturing, (again) at his wound. "You couldn't really hide something like that."

"You are right. I will remain here." Gadreel replied.

"...Okay then." Sam said, and left the room, leaving Gadreel alone.