Elijah Peterson was the brother of psychic Magda Peterson.
When Magda's powers manifested, Elijah and his family decided to split from the world and live reclusively, as they believed Magda was the Devil. Elijah appeared to still care for his sister, despite his mother scorning her.
When Magda accidentally caused the deaths of two people while trying to call for help, Sam and Dean Winchester came to investigate the cause. As the family had declared Magda was dead, Sam searched for signs of a vengeful spirit, but he instead caught Elijah and his father talking, and found out from their conversation that Magda was still alive.
Elijah and his father captured Sam and intended for Magda to use her powers on Sam, but Sam realized what Magda truly was and he insisted that Magda was not the Devil, but a psychic.
Later on, Gail decided to poison the whole family, and succeeded in killing Abraham. Gail ordered Elijah to eat the poisonous food too, but Magda intervened. Gail tried to attack Magda, but Elijah sacrificed himself to save his sister.