Lucifer

Lucifer is the second born of the four archangels, the only angels to have seen the face of God. According to the angel Gabriel, he was God's favorite celestial, but was cast down to hell for not bowing down to humanity when required. He waged a war against heaven, but was eventually banished by Michael, the eldest of the archangels.

When Sam broke the final seal by killing Lillith and releasing Lucifer, he invited the character to wander the earth freely. His motivations are quite clear, for he want to rid the world of humans, a species that he has expressed disgust towards. However, to walk the earth he needed the consent of a human vessel.

He chooses a man named Nick, who he tortures mentally by showing him repeated images of his baby's cot full of blood and his dead wife in order to force him to consent to giving up his body. This didn't work, so Nick was told the tale of how it was God's fault for not looking after his family. Nick consented if Lucifer would kill his father for some scrap of justice.

Nevertheless, Lucifer is only using Nick as a temporary measure, waiting for Sam to be seduced in the same way. In the meantime, he releases Death from his shackles in preparation for the apocalpyse.