One is our ruler. The Spirit King. He's what you would call a tyrant. No, we cannot do anything about it. Luckily he's usually neutral unless you get in his way.
— Dezadon, talking about the Spirit King.

The Spirit King is the ruler of the Spirit Realm, and most likely the most powerful spirit. Arcane and Dezadon briefly mention him during their explanation in Chapter 7.

The Spirit King seems to be a tyrannical ruler, and is powerful enough to prevent Arcane or Dezadon from doing anything to overthrow him. He has yet to make an appearance in BFS.