Faust is a descendant of the original Faustus, a man who made a pact with the devil. In Shaman King, Faust the eighth was a lonely boy who had only one friend, named Eliza. But Eliza suffered from a deadly disease that no cure, so Faust devoted his life to find one and evetually did it. He became a doctor, saved Eliza's life and married her. They lived together in their clinic in Germany until one night a burglar broke in and shot Eliza dead.
Broken Faust turned to his ancestor's black arts, necromancy, and now fights in order to become the Shaman King and bring Eliza back to life.
His spirit is Eliza herself, and his medium is her skeleton, which is always tied to Faust's arm in a chain.