He is an alternate reality version of Sam Winchester.
At some point in the past, Sam said "yes" to the archangel Lucifer, becoming his vessel. Dean, while heartbroken, attempted to try to kill Lucifer with the Colt despite the fact that it would kill Sam as well. Coming up behind Lucifer in the Men of Letters bunker, Dean shot the archangel through the back of the head with the Colt. However, Lucifer is immune to the Colt's power and quickly healed the wound. Stating that it always had to end this way, Lucifer burned Dean to death.
In Proverbs 17:3, Sam experiences a vision of the alternate Dean's death at the hands of the possessed Sam. He subsequently comes to believe that he is seeing God's possible endings for himself and Dean.
In The Trap, God reveals that all of the visions Sam has seen are really God's memories of the deaths of Sam's and Dean's from alternate realities.