Castiel
Castiel is an angel who brings Dean Winchester back from Hell following his death at the hands of Lilith's hellhounds. He tries to communicate with Dean twice after his resurrection, but fails to do so after incorrectly assuming Dean is capable of withstanding the power of his voice. He takes a human host to communicate with Dean, and tells him that he has been brought back because God has work for him to do. Dean is skeptical, but both Bobby and Sam believe the claim to be true, as there is no other being that they can find, or know of, that is capable of pulling a soul out of Hell and who is also immune to all the known methods of killing supernatural creatures. After Dean's encounter with the Witnesses, Castiel reveals to him that Lilith is releasing the 66 seals, one of which was the raising of the Witnesses, in order to free Lucifer.