Soul King

The Spirit King or Reiou is the king of Soul Society, whose existence in Soul Society is as symbolic as is absolute. The king resides in the Royal Palace that exists in a special dimension inside Soul Society that is protected by the Royal Guard. The door to the dimension housing the Royal Palace is opened using the Ouken.

It's unclear what the King actually does, other than he has a specific system of "speciality duties" which are actively carried out. The duties apparently require a very high level of competency or skill of some sort.