Royal Special Task Force

The Royal Special Task Force, known commonly as the Royal Guard or Zero Division, is the most powerful militant force in all of Soul Society. An origination made up of former Captains of the Gotei 13 who'd been promoted to this level, the Royal Guard serve as the personal bodyguards of the Spirit King and the royal family. Only captains of the Gotei 13 know of their existence, though others have been known to discover them. As former elite captains of the Gotei 13, the members of the Royal Guard have considerable power and tactical knowledge. They are the highest class of Shinigami and only answer to the King, not even the Central 46 has authority over them. While the Royal Guard is a single force, it is made up from five smaller divisions, known as the Ring of Five.

Ranks
Similar to the Gotei 13, the Royal Special Task Force has ranks. However, the ranks in the Royal Guard function differently than that of the Gotei 13. Unlike the twenty reserved ranks for officers, and the the existence of non-ranked Shinigami, the Royal Guard consists of five smaller divisions known as the Ring of Five, have only two separate leader-appointed ranks, and because all members of the Royal Guard were former captains, there are no major gaps in power between individuals.

Supreme Commander
The Supreme Commander (最高司令官, soudaishou; Supreme-Commander) is the head captain of the Royal Special task Force, and the true personal bodyguard of the Spirit King. The Supreme Commander also holds the position of being the head of the First Ring of the Ring of Five. The Supreme Commander is said to be the most powerful member of the Royal Guard, as well as being the wisest and most experienced.

High Marshal
A High Marshal (高統帥, kunshi gensui; High-Marshal) is the leader captain of one of the Rings of Five (excluding the special case of the Supreme Commander). Unlike in the Gotei 13, where captains are generally the most respected and powerful Shinigami, the position of High Marshal merely reflects one chosen for responsibility and experience in leadership. Also unlike in the Gotei 13, where captains are chosen though several means, the positions of a High Marshal are unquestioned and unchallenged. Chosen directly by the Supreme Commander, High Marshals and almost never removed or replaced.

Captain
A Captain (隊長, taichō; Captain) entirely make up the Royal Guard, including the High Marshals and the Supreme Commander. The title of captain exists in every one of the Ring of Five, and functions very differently than in the Gotei 13. The title of captain is simply an extension from their time in the Gotei 13. Because of this, there are no true superior ranks - in the sense of power - within the Royal Guard. The closest existence to division in power or ability are the classes of the Royal Ring of Five Guards, which divide the captains by their ring. Captains are divided by "levels", which are according to their ring.

Uniforms
The uniforms of the Royal Guard are both similar and different from those in the Gotei 13. Like in the Gotei 13, High Marshals and the Supreme Commander have distinguished uniforms. Like the lieutenants of the Gotei 13, the High Marshals have armbands with a crest-plate of the Royal Guard's insignia and their ring (the Ring of Three has three rings in their insignia, while the Ring of Five has five rings in their insignia). The Supreme Commander also has a silver tomoe medallion, which distinguishes him as the head of the Royal Guard.

The rings are also divided by color, albeit subtly. Like the captains of the Gotei 13, the captains have their haori from their time as a captain in the Seireitei. However, the edges of their haroi are lined with one of five different colors; Ring of Five is amethyst, Ring of Four is cerulean, Ring of Three is crimson, Ring of Two is viridian, and Ring of One is gold.

Members
All members of the Royal Special Task Force were former captains of the Gotei 13. Kirio Hikifune, who was a former Captain of the Twelfth Division, and Takashi Ishigawa, who was a former Captain of the Tenth Division, were two such members.