An Angel of the Lord, he first appears in the fourth season, and is used to introduce the theme of Christian theologyto the series. In the series, Castiel brings Dean Winchester back from Hell and frequently helps him and his brother Sam in their battles with various demons and angels along the way. During his travels with the Winchesters, Castiel develops friendships with both men. As an angel, he possesses a number of supernatural abilities, including the ability to kill demons. Initially, the character demonstrates complete devotion to God and little emotion. However, his interactions and experiences with Dean Winchester and Sam Winchester, as well as certain revelations about God and his fellow angels, have a humanizing effect on him.