Toa Hagah was the name given to the six elite Toa whose job it was to safeguard Makuta. Each Toa Hagah wore unique armor which identified him or her as a member of this special group.
In the tens of thousands of years before the Brotherhood of Makuta rebelled against Mata Nui, they often employed Toa as guards. One such Toa team was the Toa Hagah, led by a brave hero named Norik.
When Spiriah was exiled, his Toa Hagah were transferred to other duties.
The Brotherhood of Makuta chose the members of their Toa Hagah teams from existing Toa teams, and the Toa's masks were presented to them by the members of their former teams as a badge of honor.
All Toa Hagah teams carried weapons like Norik's team: special armor, Rhotuka shields, and spears.
The various Toa Hagah teams of the Makuta were not all formed at the same time. It took time to recruit Toa, and some Makuta placed more priority on it than others. If a Makuta's region was not a particularly dangerous one, then there there was not much of a rush to get a security detail. There were probably at least a few Makuta who were initially resistant to the idea of having Toa around all the time.
The Toa Hagah with silver armor did not have a different rank from those who wore gold.
The Hagah were formed to defend Makuta against dangers such as Rahi attacks and natural disasters. Makuta could deal with these dangers themselves, but didn't want to bother doing so.
The Toa Hagah were never part of the Brotherhood's army.
The Toa Hagah wore masks carved in the shape of the masks of past heroes as a tribute to them. A standard Pehkui didn't look like a Kiril, but Norik's did because it was worn by a Toa Hagah and therefore carved that way as a tribute.
The Toa Hagah's Kanohi were shaped after those of great heroes of the Matoran Universe, not Brotherhood of Makuta members before their betrayal.