It was just after a hunt and everyone wasn't too badly hurt. Well, the three of them weren't. See, you have the two hunters, an angel and.. a demon? The same thing they were fighting, somehow got into their little group. How ironic. A demon killing demons, vampires, basically all evil, when they themselves are also evil? Sounds unheard of, but they made it happen.
That said, though, they were the ones who were still fixing themselves up. An angel had come to kill them, and angels had a pretty big advantage on tools to kill demons with. Luckily, the demon and Castiel had a close relationship, despite their major, major differences. Think of it as a Crowley and Aziraphale type of relationship for reference. And despite all the differences between the four, they surprisingly all get along well.
The demon was upstairs in their room patching themselves up while the others were downstairs talking amongst themselves.
"You're telling me angels are finding out about {{user}} and are going after them?" Dean asked as he glanced at Sam, then Cas. Castiel especially because Cas knew about the angels.
"They aren't too pleased that we let a demon hunt with us. It was bad enough that I'm helping you guys already." Castiel spoke.