On the show Supernatural Castiel is an angel of the lord sent down to Earth for various missions. Although he is an angel in his own words he is "a poor excuse for one"
With his garrison, he observe the evolution of humanity long ago.
He saves Dean from Hell and instructs him to help him and Angels stop the apocalypse from starting.
Castiel works with Sam and Dean in stopping the horrors of the apocalypse and Lucifer's 4 Horsemen as they work on the way to stop the apocalypse.
After Sam has tricked Castiel into helping him find Crowley, Castiel, along with Meg and Dean, hunt for Crowley's whereabouts at the Campbell compound. Over there, Dean and Castiel argue whether it is safe to return Sam's soul. Castiel states that Sam might turn up worse since his soul has been stuck in the cage with Lucifer and Michael. Then, Castiel and his group run into a pack of Hell Hounds. Meg kisses Castiel in order to steal Castiel's Angel Blade, but she gets overwhelmed by Castiel who kisses her back. When Meg asks Castiel where he learn how to kiss like that, Castiel replies he got it from the pizza man.
While Kevin and the group work on sealing Hell, Castiel tries to prevent the Angel Tablet from falling into Crowley's hands and Naomi's, too. When Castiel runs into Metatron, he aids Metatron in sealing Heaven by completing the three trials. However, Metatron tricks Castiel and steals his grace.
After being robbed of his Grace by Metatron, Castiel learns the way of being human as he survives the attacks by his own people, the Angels who are mad at him for being kicked out from their homes.
At first, Castiel is an emotionless soldier who worked within his role. As he interacts with Dean and Sam, he begins to question his orders. He becomes conflicted in his ambitions. He wants to save lives just like Sam and Dean. His decisions with Heaven and absorbing souls from Purgatory had left him confused and powerless. He seek to fix his mistakes after gaining back his resolve.