In the series, Castiel brings Dean Winchester back from Hell, and helps Dean and his brother Sam in their battles with various demons and angels. As an angel, he possesses a number of supernatural abilities, including the ability to kill demons with his touch.
Castiel typically displays very little emotion and always exhibits an extremely somber disposition. Although it has been suggested angels possibly do not possess the ability to truly feel emotion, Castiel frequently exhibits what could be referred to as a friendly affection towards Dean. This attachment and growing ability to feel even causes him to be "demoted", as his superiors fear emotions are clouding his judgment.
Unlike a stereotypical portrayal of television angels, Castiel does not always help people, and is willing to kill innocents if needed.