A Brother-Sergeant is basically what his name implies; he's a black sergeant. Wait, no, we got that wrong...
A Brother-Sergeant is basically what his name implies; he's the Space Marine equivalent of a sergeant in the Imperial Guard (except with thicker armor, bigger guns, higher statline, and pauldrons), an Eldar Exarch (debatable, this point is), and the Shas'ui of the Tau. Basically, he's the guy in charge of a squad of SPESS MEHREENZ, a step above Battle-Brother, and a step below Lieutenant. Typically, he's there by virtue of duration of service, or by virtue of the previous sergeant getting cut the fuck apart by Tyranids, which seems to be the most popular way for them to die in canon. They're usually the guys who, if they don't get killed early in the story, have plot armor that is thicker than the armor on an Imperator Titan. This typically manifests in the most cheesy (and admittedly awesome) ways, one of the most popular of which is their ability to charge into thousands of enemies with guns who fire ineffectually or inaccurately at the literal giant adorned in vividly-painted armor, and once in close quarters everything dies. EVERYTHING. Including Chaos Marines.
Weaponry and wargear
Your average Sergeant comes equipped with a bolt pistol and a chainsword because of tradition. A Sergeant's rank is represented on his power armor by the Iron Skull, which dates to the era of the Legiones Astartes. One piece of advice recommended by the Codex Astartes is to identify the Sergeant with a red helmet. This originally dates back to the Great Crusade where a red helmet was a mark of censure under the Ultramarines Legion, something they borrowed from the Night Lords Legion's use of sinners-red gauntlets to mark condemned Legionaries. However, during the Battle of Calth, Sergeant Aeonid Thiel of the 135th Company, who had been marked for censure for forming theoreticals against Space Marines, proved instrumental in leading the counter-assault on the Word Bearers, so Roboute Guilliman ordered all Sergeants to repaint their helmets red, saying it would be a mark of honour.
There's a few different kinds of Sergeants.
- Scout Sergeant: Basically a Space Marine who decided he prefers being sneaky. Typically these are highly-experienced sergeants who either favor stealth and subtlety, and prefer to educate the next generation of their Chapter rather than fighting alongside the glory hounds of the rest of their Chapter, or who just like growling and snarling cynically and pessimistically about everything without having to worry about someone with actual experience telling him to shut the hell up and quit being such an emo git. It can also be a combination of any or all of these factors. They don't wear power armor but they do wield sniper rifles so fucking huge and powerful that it'd make the modern day M82 Barrett look like a fuckin' peashooter, so...yeah, who the fuck needs armor when you can reduce a Warboss's head to mush from 5+ miles with the pull of a trigger?
- Veteran Sergeant: Well, Brother-Sergeants are already veterans of hundreds of battles, so these guys are veteran veterans. They typically are part of the Chapter's 1st Company and have received their Terminator Honors, though they may not actually wear Terminator armor themselves. They're often seconded to other companies' squads to impart their centuries of wisdom onto the lesser-experienced Astartes in the Chapter to keep everyone up to snuff. They also tend to be attached to Command Squads. This is often actually a dead-end sort of rank to have; Veteran Sergeants don't typically advance to Brother-Captains, though this largely because the Brother-Captains don't take to the field as much, whereas Brother-Sergeants ALWAYS take to the field if their squad is deployed, thus making them more the aspect of "might" in the Chapter, where the Captains are the aspect of the "mind," if you will. They get better equipment in general and it's a foregone conclusion that if one of them is bearing down on you...well, you're fucked, should've prayed harder. Or not have been born a filthy xenos.
- Terminator Sergeant: A Sergeant in command of a Terminator Squad. Hard as an Imperial Guardsman's standard-issue adamantium balls but not quite as manly, Terminator Sergeants are still beastly as hell. Funny thing is, though, this beastliness doesn't actually tend to show up much in fluff. In fact, they largely serve as the faceless icons of unimaginable toughness who get cut apart like a katana through paper. Their place is that of the Eldar Avatar, when Space Marines have to die instead of Eldar in order to give the big-bad (or the protagonist) street cred.
On the tabletop
These guys are basically the one guy in the squad who looks a little different than the others for whatever reason you want to think up. Generally-speaking, they aren't very different from the other units in the squad other than that they typically have one more attack (without special wargear taken into account) and Leadership, and have all that
7th8th edition character rule and challenge stuff going on. Nothing very remarkable, for the most part.
|Forces of the Blood Angels|
|Command:|| Apothecary - Brother-Captain - Brother-Sergeant - Chaplain |
Chapter Master - Librarian - Sanguinary Priest - Techmarine
|Troops:|| Assault Squad - Devastator Squad - Sanguinary Guard - Scout Squad |
Tactical Squad - Terminator Squad - Veteran Squad
|Death Company:||Death Company - Death Company Chaplain - Death Company Terminator|
|Walkers:|| Contemptor-Furioso Dreadnought - Death Company Dreadnought |
Dreadnought - Furioso Dreadnought - Librarian Dreadnought
|Transports:||Drop Pod - Razorback - Rhino|
|Vehicles:|| Baal Predator - Land Raider (Land Raider Crusader - Land Raider Phobos |
Land Raider Redeemer - Land Raider Angel Infernus)
Predator Tank - Vindicator - Hunter - Stalker - Whirlwind
|Flyers:||Stormhawk - Stormraven - Stormtalon - Storm Eagle - Thunderhawk|
|Allies:||Space Marines - Primaris Marines|