Him

 The Demon Boss is the leader of all Demons and the Demon World. Their name is not yet clear but is mentioned in several instances:
 * 1) Sister Krone had a private talk with Norman and Emma then she mentions the Demon's name.
 * 2) The Demons have a tea party that offers something to the Demon boss.
 * 3) If the kids break the Promise, Mujika says they'll make an enemy of the Demon Boss. But Sonju said that the Demon Boss was their enemy from the very start undefined.