Zachariah is Castiel's superior. He is a bit wary of humans, describing them as 'smelly things'. Despite this he has proven himself to know a great more about humanity than his inferiors, and even acts much more human, with a sharp wit and cheerful exterior.
After Dean tells Castiel that the task set to him is too much, Zachariah teaches Dean a lesson by giving him and his brother new, separate lives. Despite this, the brothers find one another and team up to deal with a haunting. This experience proves to Dean that he is a hunter in his heart. Realizing that the chance to make a difference is something most people never get, Dean embraces his role.
In "The Monster at the End of the Book", Zachariah contacts Chuck after the latter has an unknown but dark prophecy concerning the Winchesters, warning Chuck against alerting the brothers. When Lilith nears breaking the final seal in "Lucifer Rising", Zachariah and Castiel imprison Dean in an idyllic room in order to keep him safe. After repeated questioning by Dean, Zachariah admits that Heaven is allowing the demons to break the seals. Confident in their ability to defeat the forces of Hell once Lucifer is unleashed, they see this as a means of finally attaining paradise.
When Lucifer is freed, Zachariah attempts to convince Dean to become the vessel of the archangel Michael, going as far as torturing Sam and Dean, threatening their lives. However, Zachariah is shocked when Castiel, who had been killed by the archangel Raphael, appears. To Zachariah's horror, Castiel hints at his belief that God has been interfering. Castiel then orders him away, and Zachariah acquiesces. In "The End", he reappears to Dean and drags him to a dark future, where Sam has become Lucifer's vessel five years into the Apocalypse. Zachariah tries to convince Dean to take his role as Michael's vessel as the future Dean had refused in the past, but this backfires; Dean instead joins up with Sam again in order not to let his brother lose his way.