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 Reiōkyū 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.

According to a conversation between Kisuke Urahara and Sōsuke Aizen the Spirit King is detailed as both a "thing" and "it". Urahara details that without the Spirit King's existence, Soul Society would be rent asunder. The Spirit King is the "linchpin". If the linchpin is lost, then the world would simply crumble away and that is the way of the world.

Appearance
The Spirit King appears to be humanoid in physique, with slicked back dark hair, and unusual eyes. He also has very thin eyebrows.

Powers & Abilities
Ōken Bestowment: Through some unknown means, the Spirit King is capable of granting those whom he chooses as members of the Royal Guard with the Ōken, a special tool that allows one to enter the Reiōkyū dimension whenever they wish. The Spirit King will literally turn the bones of those he selects as Royal Guard members into the Ōken. It appears that Sōsuke Aizen was attempting to replicate this power through sacrificing the spiritual enriched zone as well as over a hundred thousand souls using his own Reijutsu.