Castiel is a man who has many names. Some call him the angel of the Lord's grace, while others call him just Castiel. He is said to have been one of the first angels to have ever existed. He was the first angel ever created, and at that time he was given complete dominion over all of Heaven and Earth
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From then on, whenever an angel was created, their sole purpose became to serve God and carry out his commands. Castiel was the only angel until he met Dean Winchester.
As part of his service to God, Castiel has been sent down (on Earth) to watch over Dean Winchester. He hasn't always been sent down to earth, though; in fact, before meeting Dean he spent centuries watching over long dead prophets in Heaven.
Many angels believe that he has grown tired of this task and that he will return to Heaven before too long (a belief that is also held by Sam Winchester). Yet even if this is true, Castiel has made it clear through his actions that he intends to stay with Dean for as long as it takes for him to get his life in order.
Castiel believes himself to be human; actually, he believes himself to be "simply" human. It isn't until Dean tells him about what happened when he was killed by Michael's grace that Castiel realizes just how much he truly is not human.
Before then, Castiel believed in heaven above all else; after that revelation however, it seems that nothing will stop him from protecting Dean - not even Heaven itself.
Castiel lives alone in his small apartment in Lawrence, Kansas. The only thing in his apartment aside from furniture is an old-fashioned clock tower that seems to have a mind of its own and a sign reading "No Trespassing." Despite this sign however, a pair of demons have set up a permanent residence inside of Castiel's apartment - one demon being the very same demon who tortured him for thousands of years prior to his death and another being a demon whom Castiel met during his time as "just" a human named Warren Trask - Warren Trask being the demon who tried to convince Castiel not to let Dean go with him on his mission of finding God's vessel on Earth.
In spite of the demons living inside of his apartment with him on a permanent basis, Castiel does not attempt to kill them or send them back into Hell upon their discovery - instead asking them for help on any number
