Missouri Moseley is a psychic for many years. Right after the fire that took Mary Winchester, John connected with her and she helped him see behind the curtain, so to speak.
Eric Kripke had originally wanted to bring Missouri Moseley back in the show for the season finale Devil's Trap. However, there was a scheduling conflict with the actress Loretta Devine, so they created a new father figure for Dean and Sam - Bobby.Supernatural Paleyfest 2011, interview by Hollywookiee.com