Jiralhanae Bodyguard

The Brute Bodyguard is a Brute (Jiralhanae) rank/specialty. They are responsible for the protection of their tribe's Chieftain, they are also responsible for the enforcement of tribal law. Whenever there is a Chieftain nearby, you will encounter Brute Bodyguards which look very similar to Captains. Bodyguard Brutes are also seen to be protecting Truth on the level The Covenant. They are likely the replacement for the Honor Guard being that Tartarus also had Honor Guard protection. Brute bodyguards seem to favor Covenant weapons such as the plasma rifle and carbine rather than their own native tech.

They come in two variants: A more common, silvery-blue (with red underarmor) variant, and a less common, silver armored (with red underarmor) variant. Whether these variants signify differences in behaviour or strength is unknown as of yet.

Brute Captain Ultra vs. common Brute Bodyguard
The Brute Captain Ultras and the common Brute Bodyguard look well the same, but there are slight differences in color. The Captain Ultra has a more cyan-like power armor, and has the standard underarmor. The common Brute Bodyguard, however, has a more indigo-like power armor and has a red underarmor.

Speculation of difference in the two bodyguard ranks
Since the silver bodyguards have only been seen guarding the Prophet of Truth on the level The Covenant, it is speculated that the silver bodyguards are tasked to guard prophets, while the common blue variation guards Chieftains.