Sam knows it long before Dean does, if Dean ever figures it out at all. It’s not in the way Dean cares, but in the way he hurts, the way Cas’ betrayals cut him in a way that even Sam’s never have. Cas is something worse than family, and Sam knows exactly what that means.
Cas has always known it, never questioned it. He can’t know for sure, but sometimes he feels as though God created him to raise Dean from perdition. Whatever destiny he writes for himself, now that the books have all been burned, Dean Winchester will always be his origin story. When Dean prays to him, even calls his name, the words burn white-hot on the back of his eyelids, and something deep within screams go to him.
Sam has prayed to Cas a thousand times without answer. Cas has heard them all, but the effect is something else entirely.
Sam doesn’t know where to begin, the night he asks God for forgiveness. He has more blood on his hands than can ever be cleansed, more lies and betrayal and anger and violence than he can confess in the rest of his lifetime. As he sits in the box he thinks of Dean, and a hundred things he’s done to him, and, briefly, he thinks of Cas. The ninth circle of Dante’s hell is reserved for treachery, and so Sam decides to start there.
It is the night after the angels fell now, and Cas feels like a deaf mute, learning to perceive the world all over again. His mind searches out for the bright angry spark that is Dean Winchester, but those avenues are closed to him now. He walks, because it is all he can do. His body will become weary, and he will need to rest, but he doesn’t quite understand that, not yet. Something in the pit of his stomach, something strangely familiar, tells him to follow the road, and so he does.
It is the night after the angels fell, and Dean has gone looking for Castiel, looking for something, looking for answers. Sam is in the bunker, and he may be dying, but he is not afraid. He thinks of the angels falling from the sky, and he thinks of Cas.
Come here, Sam Winchester prays, though he doesn’t know if Cas can hear him.
Follow the road says the warm feeling in Cas’ stomach, and he does. His feet are bleeding, but he doesn’t realize it, not yet.
“Check outside,” Sam tells Kevin, one week after the angels fell, because he knows Cas is there, though he can’t possibly be.
Sam had known it long before Dean did, not because of the way Dean cares but because of the way Sam does. Cas had always known it and never questioned it, except for when he did, when Sam’s prayers made the pit of his stomach grow warm.
Cas stands, swaying softly from side to side, weak from hunger and fatigue. His shoes are stained with blood and dirt. Sam knows that Cas is a man, and Cas knows it now, too.
Sam swallows hard. “How did you find us?”
“You called for me,” Cas says.
All the books have been burned, the ashes scattered on the wind. It is time to rewrite his origin story.