Being the go-to girl for folklore facts both large and small for a great many people, I've been the target of a lot of 'what the...?' questions over the past few weeks. So I thought I'd take a moment to address the biggest of them. Because it's fun!
So Castiel. Who is he? Where did he come from? Is he a shiny new angel, or is he something out of the existing mythology? Who knows? Well, presumably Kripke knows, but since we can't really get him to give a straight answer...
After going through six reference books containing lists of angels, I was unable to find anything that mentioned 'Castiel' as an 'official' member of the heavenly host. Sorry, dude. I was, however, able to locate two potential candidates. Remember that many angels have names that are sort of, well, funky by our modern standards -- there's a reason that my version of Sahaqiel calls herself 'Magpie' most of the time, and it's not just because she's hiding from Heaven. It is thus entirely believable that either Kripke has 'updated' Castiel's name, or that he's going by a source that predates the current 'common' spelling. So, our candidates...
CAPHRIEL.
Caphriel is one of our weekday angels -- much like we used to get underpants that had the day written on them, God used to keep a bunch of angels around to fill basically the same role. Caphriel got ownership of Sunday, the day of rest. None of my books really said anything else about Caphriel, presumably because when your purpose is 'make sure the laziest day of the week keeps happening,' you take a lot of naps. Caphriel is nicely undefined, and could thus be tacked on to almost any duty.
CASSIEL.
Now, really, Cassiel seems like a much more likely candidate as the model for our dear Castiel. For one thing, he's a pretty regular player in texts on angels and the angelic host; it's thus easier to find references to him and his exploits. (His name is alternately spelled as 'Kafziel,' which is really, really easy to mangle into 'Castiel.') He has an actual job! He assists in leading one of the larger choirs of angels, and he's generally taken to possess a fairly high rank within the host as a whole. Like Caphriel, he 'owns' a day of the week; his day is Saturday, the original Sabbath, making him a much more influential angel. His official title includes 'angel of tears and temperance.' Sounds like a good match for darling Dean, now, doesn't it?
Given what we have to work with, I'm going to vote for Cassiel, and assume that 'Castiel' is an alternate spelling/pronunciation of his name. He's a good fit, and since Kripke will usually go with existing material when he can, he makes sense. Why use Lilith and then invent an angel?
And that closes our thumbnail folklore lesson for the day.