Soul King

The is the king of Soul Society, whose existence in Soul Society is as symbolic as it is absolute. The king resides in the Royal Palace that exists in a separate and special dimension inside Soul Society that is protected by the Royal Guard. The door to the dimension housing the Royal Palace is opened via the Ōken.

It's unclear what the Spirit King actually does, other than he has a specific system of "specialty duties" that are actively carried out. The duties apparently require a very high level of competency or skill of some sort. The Spirit King and the Royal Family are apparently completely indifferent to governing Soul Society; the Spirit King has given the full control of government of Soul Society to Central 46, thus being uninvolved with the day-to-day affairs of Soul Society.

Both Aizen and Urahara have referred to the Spirit king as an object, rather than a person. According to Urahara the Spirit King is the "lynchpin" of Soul Society, without which it would crumble.