Dean forgets, sometimes, what Castiel is. The "baby in a trench coat" with startlingly blue eyes is an angel of the Lord, a warrior of God. A creature who has been alive for millennia, who has watched the rise and fall of civilizations. Of course, he knows what Castiel is in an academic sense, but when said baby disappears with a fluttering sigh, Dean is far more likely to be annoyed than he is to feel awe at the vast power of his friend. He appreciates the convenience of Castiel's ability to heal with a touch, or to travel thousands of miles away and back again in an instant, or to smite demons in a blaze of holy light.
It is in the terrible aftermath of a fight, when Castiel's vessel has been sliced or stabbed by an angel blade and a bright glow emanates from the wound, a tiny glimpse of the angel's true form, that Dean remembers that the frumpy clothes and frowny face are a façade, a suit that Castiel wears to make himself available for human perception. Dean knows the terror and awe of the might of an Angel when Castiel moves through a rain of bullets without pause, when he pulls a knife from his sternum with an annoyed sigh and a look. In Dean's dreams, when he is trapped back in hell and a slowly growing light surrounds him, cradles him in warmth and releases him from his bonds before he rises from the loosely packed earth of his grave and he rises from his restless sleep. When he sees the shiny raised skin of a handprint on his shoulder, and later, when that scar (all his scars) is gone and he feels a strange emptiness at the sight of blank skin in the mirror. In dark stolen moments in cheap hotels, Castiel kisses him with chapped lips and Dean is struck by awe when Castiel rises above him, towering and sweaty, and Dean feels like if he just stared a bit longer, if he just looked harder, he would be able to see Castiel's wings stretched wide and black.