Bobby Singer is a hunter who existed in an apocalyptic world. He is the alternative reality version of Bobby Singer.
In All Along the Watchtower, after entering a post-apocalyptic alternate reality through a tear in the fabric of space and time created by Lucifer's son, Castiel is saved from a Tempter Demon by an alternate reality version of Bobby. While Bobby generally kills both angels and demons on sight, he recognizes that Castiel is different from the angels of his world and spares him. Castiel later brings the Winchesters back to the alternate reality where they are overjoyed to see Bobby who has no idea who they are. Bobby tells them that the only Winchester he knows is John Winchester from a story told by Mary Campbell of the man she loved and lost. The Winchesters learn that in this reality, they were never born and thus couldn't save the world. Bobby then tells the Winchesters about the angel-killing bullets his machine gun is loaded with and lends it to Dean for use against Lucifer.