Guide
Guide : the person primarily in charge of the storyline and adjudicating the rules.
Think of the Guide as a kind of referee who makes sure that all the players get a fair crack of the whip during play.
The Guide writes the basis for the stories that the player characters will attempt to influence, plays all the minor bit-parts, villains and other NPC's, and makes rulings where the standard rules given in this manual do not seem appropriate.