The Brute Bodyguard is a Brute (Jiralhanae) rank/specialty [1]. 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.