(WARNING!!! Spoilers Follow)
Gadreel, portrayed by Tahmoh Penikett is an angel wanting to redeem himself. He hides behind a false name (Ezekiel) and offers his help to Dean by possessing Sam, to help heal him.
Gadreel is an imprisoned angel who allowed the serpent into the garden. He is soon under the influence of Metatron, in hopes that his name will be redeemed. However, Metatron only wishes to possess power for himself.
Gadreel does redeem himself by sacrificing himself so that Castiel could defeat Metatron and restore Heaven, hoping that he would be remembered for helping give Heaven a second chance.