Gabriel could see what Dean and Sam couldn't. Human vision was too limited to see the lackluster glow of damaged heavenly power, the faint pulse of a light close to burning out. Castiel's wings, once massive, invisibly taking up space that the Winchesters never noticed, were conspicuously absent. His true form was crammed inside this vessel, packed tight and tearing apart Jimmy Novak's body from the inside while simultaneously healing it — but the human soul that had been inside it when Gabriel last saw Castiel was gone. Castiel's angelic essence was no longer so awesome and hurricane-like in its power that it felt like the vessel wasn't part of him. To an angel, apart from his flickering stolen grace, Castiel looked neither angel nor human, but somewhere lost in between.
He stepped inside, no longer needing to scrunch in his wings (not that it mattered if he did; the physical plane really did nothing to obstruct them).
"Hello, Gabriel."
Castiel had an instinctive sense of reverence for his brother. The archangels were stronger, older, more powerful, and even after Gabriel had been missing for so long, after the chaos in Heaven, Castiel still felt an inherent, ingrained sense of respect. Gabriel was, and would always be, his superior, even if Castiel had defied the angelic hierarchy years ago.
He squinted at Gabriel, looking him over. How damaged must Castiel look in comparison?
"I didn't want you to learn any of this from the Winchesters," he said, closing the door behind him. "The Apocalypse was prevented, but what came after was worse."
He stopped himself, frowning slightly. Was that a good way to start a conversation? Castiel had always been sort of bad at that, diving in with utter seriousness and forgetting social niceties.