At this point on the show's run, Castiel could arguably be considered one of the show's leads. And that would be fair. But at Supernatural's heart, it will always be a show about two brothers, with Castiel sort of on the second tier. Not quite a sidekick, but not one of the show's two leads. Both Sam and Dean have some depth to them, but they fall into neat little archetypes. Not so with Castiel. How do you even classify a character who is an angel (except when he's not an angel) who first appears on the show as one of God's little yes men, then gradually begins to question his faith and eventually throws his lot in with the humans? The writing for him is consistently strong and combined with Misha Collins' compelling performance creates a fascinating character study, filled with a developmental arc that has been a joy to watch over the years.