"Demons hide in plain sight. There eyes won't show until they announce themselves." Cas scrubs a hand over his face, weary, but none of it Kyle's fault. He shakes his head, dismissing Kyle's need to explain. "You couldn't have known."

He doesn't blame him for any part of this. Kyle's far from the first or last to be charmed by Dean Winchester, Dean's demonic state is news to Cas as much as it was Kyle. His friend's guilt is misplaced.

"It's alright, Kyle. Truly."

Cas intends for the smile to be reassuring, though it's more hapless. The words, at least, are sincere. As for restoring Dean, the angel's earlier assurance was mostly wishful thinking and bullshit. A Knight of Hell is a whole different creature from what Dean was before.

"I don't know. There's possibilities, maybe." None of them will address the Mark as well as the demon. "A ritual with human blood, perhaps, but it won't fix the root problem."

Sam's trial with Crowley does come to mind, back when Castiel had been busy being tricked by Metatron into booting all the angels from Heaven. But that still doesn't solve the Mark.