It doesn't even feel like him anymore.
Sometimes, the word sounds strange in his own ears.
It's not as if anyone from home ever calls for him anymore.
It's not as if anyone here ever uses it when they do.
Early on, he demanded respect, made threats (he's regretted, ever since the first night he sat at Dean's bedside and watched the nightmares steal his sleep) – and the boy's only response was to defy his every order, disregard his mission, and give him a casual nickname that seems to have stuck.
Even the other angels had started calling him Cas, before...
Well… before.
When he asked about it once, Sam told him it was affection; even back then, Dean saw him as a friend, someone on their side in the whole crazy Apocalypse mess. Sam explained that that's what humans do, when they get close to someone, when that someone starts to feel like family.
Secretly, Cas wonders if it wasn't just a way of telling him how very unintimidating he really was.
He knows something the Winchesters aren't really aware of – and that's that names have importance… significance… power.
And he hasn't had a shred of that, not since the moment Dean Winchester dismissed his name in favor of the one he gave him. It's as if Dean claimed him in that moment, locked him into an unbreakable bond, because he's never been able to do anything but love the man ever since. He serves Dean with a dedication that frightens him sometimes, so intense it's almost blasphemous.
He sometimes thinks that Hester was probably right.
Perhaps it goes back even farther. Perhaps "Castiel" really was lost the moment he touched the soul of Dean Winchester.
It doesn't matter if it's true, not really.
He'd rather be "Cas", anyway.