Castiel

Castiel, a fallen angel, was responsible for taking Dean Winchester out of hell, by God's order. It was his job to prevent Lilith from breaking the 66 seals that, once broken, would release Lucifer from his cage. Once Castiel learned that the angels wanted to free Lucifer, he defied his angelic brothers and died trying to help Dean keep Sam from breaking the last seal. Castiel doesn't tell anyone that he's an angel, he keeps it a secret and acts as if he were human...only Dean, Sam and Bob know this secret.