Him

The Demon Boss (also mentioned as "The One") is seemingly the leader of all Demons and the Demon World. His 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.