Demons
"Sam!" Dean yelps as he walks into the motel room and sees Harry and Crowley sitting across from each other, the demon nursing an ever-present glass of scotch and the wizard twirling a bottle of butterbeer (which Dean is disappointed to say does not actually contain alcohol, though Harry insists it's good for getting house-elves drunk).
"I know," Sam sighs from his bed, where he's researching on his laptop. "He won't go away."
"Your hospitality is touching," Crowley snarks.
Dean scowls. "I'm warning you, Crowley, one wrong move..." He holds up his flask of holy water and Crowley tips his glass in surrender.
"Yes, yes, all right. I just heard about your delightful new companion, thought I'd stake out the territory for myself." The demon nods to Harry.
"You never told me the King of Crossroads was British!" Harry says cheerfully to Dean, who stares at the wizard.
"Scottish," Crowley grumbles. Harry dismisses it with a shrug. "I 'spose you went to Hogwarts, then? Gryffindor?" Harry nods. "Figures."
"You know about Hogwarts?" Dean asks blankly.
"'Course I do. Got bloody expelled from the place." Crowley scowls even as Dean's jaw drops to the floor. "I mean, seriously, it's not my fault, the death of the stupid Mud-"
"Oi," Harry warns sharply, going for his wand.
"Muggleborn," Crowley corrects hastily. "She found the Chamber of Secrets, 'course the girl got herself killed. Never mind that I only told her how to get in, that apparently counts as murder. They snapped my wand and everything," he adds mournfully.
Dean decides it's better not to know. His head's starting to hurt from the new information about Crowley, as it is.
For those who are confused as to why Crowley wasn't in Azkaban, he ran away and disguised himself as a Muggle tailor after he was expelled. His son hated him out of jealousy because he was born a Squib.
...shh. Just go with it. I'm making it up as I go.