Ratskins are only mentioned in the fluff of Necromunda and are never seen, it's actually the bad ones who get any screen time. That's because otherwise there would be a swathe of neo-buddhist/pacifist armies organising an exodus from the tabletop each battle, which doesn't really work in the setting, because 40k is grimdark like that.
For their own part, the Ratskins call themselves the "Kuloq" which means "The People" and that their origin story was that their deity "Ku" got lonely at having no-one to share his planet with, and so: "in an act of grief and longing he laid himself down upon the planet and allowed his essence to seep into the soil" and that the people are the descendants of his left hand... I think that's enough said about that.
Unlike the raving bands of Scavvies who share the lower territories of the hive, the ratskins are NOT characterised by mutation, cannibalism or violence. As described, mainstream ratskins (largely derived from a pastiche of Amerindian and other tribal stereotypes) are probably the closest thing to a society in the 40K universe who's ethics we might broadly approve of if you like pacifistic, Chaotic Good-ish people who mind their own business and revere, uh, "nature" (well, really, think about it, accepting the adage that humans are a part of "Nature", then after tens of thousands of years, the remnants of the Hive that are left unto themselves are as close to "Nature" as there is.
Except for the other contender for "nature" Heresy! Who let Lorgar in here?)
They do not interact or trade with the hivers in any way, finding their customs and behaviours bizarre and strange. "Normal" Ratskin tribes do not engage in violent conduct, arguing that the soul of a slain enemy will return to seek vengeance, thus if they ever encounter "normal" citizens the Imperium they will get the FUCK out of dodge. You might never see them but they were there. Some Ratskin tribes do engage in violent conduct, but they are a minority and were added as optional extra rules to differentiate between different Ratskin tribes.
Away from the Preachers of the Ecclesiarchy, the ratskins do not share any of the same beliefs common amongst the normal citizens of the Imperium, instead they have formed an animistic form of shamanism in relation to the hive. Coming to worship the spirits of the hive they treat the city as a living ecosystem prone to moods and tempers.
The hive guilds have placed bounties upon the ratskins, and they are not welcome amongst imperial society, mostly because this hippy crap offends the Emperor.
Though they live primitive, self-sufficient lives the Ratskins do consider Archaeotech to be sacred and while they do not use ancient technology, they maintain that sites containing archeotech are favoured by the spirits. Thus locations containing archaotech are often the only static territories held by the nomadic ratskins.
Apparently, for unknown reasons the Ratskins have a higher than average occurrence of psykers in their communities. Those who develop powers are directed to become Shamans or Shaman-Adepts.
Despite being respected figures in their tribes, they don't take leadership positions, so if a player loses his gang leader the Shaman gets excluded from the running to the top job.
They have their own psychic power table, although they insist that their powers come from the Hive itself (and not the Warp), which has led them to be held under close scrutiny by the Ordo Hereticus.
They also get the job of caring for archeotech that the tribe finds, and in rules terms this means they get to squirrel it away to curry favour from the spirits each time they do so, granting them an additional power from the Spirit Lore power chart.
The Totem Warriors are those amongst the ratskins who seek to purify themselves in the sight of the hive spirits. For this they are revered by the badskins as well as the more peaceful ratskin peoples. They are seen as wandering warrior-pilgrims who spend their time on quests or in prayer-trances to the spirits of the hive.
In their devotion to the spirits, the totem warriors believe that by slaying an opponent in close combat, they can absorb their enemy's soul, thereby avoiding the spiritual vengeance of their opponents as well as adding to their own power.
While Totem Warriors eschew ranged combat in all cases, their purification rituals and superstitions do invest themselves in their melee weaponry. When in the hands of the totem warriors they can inflict damage far beyond that of their normal type. Like the Shamanic powers, the totem Weapons are infused with the power of "spirits of the hive" (sounds like how Space Wolves view their psychic powers)
Ratskin Renegades / Badskins
Aside from the peace-loving, nature-worshipping angle, Ratskins are also depicted as gambling, drinking, whoring good-for-nothings who mosey into the saloon and get evil looks from the inbred backward locals.
Those ratskin tribesmen who come into contact with the hivers are shunned, as they are considered to be "tainted by civilization". These "Badskins" often become vagrants and troublemakers, though some do find legitimate work and can be hired by gangs in the tabletop game to serve as scouts, guides or trappers.
Occasionally drift together to form groups of "Ratskin Renegades", becoming violent and looting and stripping hive territories dry before moving onwards.
These ratskin renegade tribes are organised much like normal ratskin tribes, led by a Chief who is elected from amongst the warriors in the tribe, accompanied by the warriors and unexperienced braves armed with whatever they can loot or purchase from black-market dealers.
In a nutshell this means you get to use Ratskins Renegades as a playable faction in Necromunda.
There is an another travelling warrior amongst the Ratskin tribes, the Savage. Though unlike the Totem Warrior they normally do not remain attached to gangs and only join in after receiving their hired gun fee and then leave after a short period.
This is fine because Savages can start with much higher statlines than Totem warriors (who progress in XP along with the rest of the gang). So they make for a good method of quickly buffing your force if you need an extra pair of hands.
What also distinguishes the Savage from the Totem Warrior is how they behave. The Savages are NOT seen as holy wanderers, rather they are seen as the absolute worst of the Badskins, who not only have been "tainted by civilisation" but want to see all of the hive dwellers get slaughtered and fall into unstoppable rages in order to get this done. In rules terms they focus entirely on close combat, and gain the two-weapon bonus even if disarmed, they also cannot be parried in close combat on a 4+ due to the sheer ferocity of their attacks.
Finally, if they take an opponent out of action (models don't usually die straight away in Necromunda) then the Savage can descend on them and attempt to give them a rusty-bladed scalping if they can't get away in time, which will basically give the opponent's character some form of disfigurement (and a permanent penalty like being blinded or deafened... or DEAD).
Eventually, Fanatic Magazine eventually settled down and wrote out what makes different Ratskin tribes distinct from each other, which gave them distinct rules benefits.
Are the largest tribe, they see themselves as the purest of the people, so they don't actually have any distinct rules differences from the rest of the Ratskin tribes. They are the ones who forbade contact with outsiders which is why Ratskins are not often seen.
One tribal rite that is distinctively theirs is that young males must make their way into the underhive, strangle a rat to death with his bare hands, then eat its heart. Now considering Hive World rats can grow to the size of horses, this is pretty MANLY, then he gets to keep the rat pelt and return to his tribe as one of the people.
Tech lovers, tend to know where most of the Archeotech is stored. They also have tribal rites dedicated to the worship of these relic-treasures and therefore take amazingly good care of what they find.
They have a large stash of lasguns, and manhood revolves around receiving your very own.
What makes this tribe different is that they have no apparent psychic potential, therefore their gangs may not include Totem Warriors, nor may they include Ratskin Shamans who are low-level Wyrds. The positive side to this is that each gang member starts with a free stub gun instead of a knife, and players get access to the "Techno" skills table, which is pretty sweet.
The violent tribe, they daub their bodies in luminescent fungus-paint and protect their holy places from invaders. Like all Ratskins, they believe that killing others makes them return as vengeful spirits. The Nassaq believe this is a good thing for them, since they see the Imperium as the superior foe, so they figure that when they die they can come back to haunt the Imperium and be a pain in the ass.
They have more Totem Warriors than most other tribes, represented in the rules as up to half the total gang and they are completely immune to fear at the start of a battle (though this fades off after they spend one round in combat). However, they cannot take Shamans.
Isolationists, even amongst Ratskins. They occupy the dark, secret places that nobody else know about. They worship the spirit of the "Stones" which are pretty much the giant stone pillars that hold up the Hive (and essentially their whole world) above their heads. This isolation spent in the dark has rendered them poor combatants, reducing their WS or BS by -1 (determined for each ganger) and has given them ghostly pale skin. Though their adaptation to their environment means they all start the game with Toughness 4 while their increased spirituality means they can take a Shaman Adept juvies into battle with them and potentially have two psykers (though the Adept must earn his first power by levelling up) something other Ratskin's can't do.
The Caller / Rat God
Nothing to do with the Horned Rat.
The Ratskins have one champion, called "The Caller" who was supposedly going to unify the Ratskin tribes and drive away the Imperium so that the planet could be returned to its rightful people. Doing this, he managed to collect together a small army of Ratskin tribes as well as Scavvies and other outcasts.
The Rat God is his mount, supposedly resurrected from the body of a Rat God lain dormant in the underhive for centuries. Essentially a giant undead daemon-rat.
The Caller is pretty much the end-boss of Necromunda, having a higher rating than any other character in the setting and can only be allowed in the game if: A. the group agrees to it, and B. the player using him has the worse gang rating in the group.
Together though they are hard as nails, the Rat God has an excellent stat-line bolstered up by an Invulnerable saving throw (rare things in Necromunda) and its close combat attacks with its teeth automatically wound and completely ignore armour too, as well as chew through vehicles automatically.(which amounts to jeeps and bikes in Necromunda, not Land Raider battle tanks) What a boss... the Caller also has a pretty good statline and what amounts to a psychic Bloodmare Stone which renders all allied Ratskins resistant to pinning or can be splurged out on a searing energy blast that FUCKS enemy models and if they survive they automatically shit a brick.
He was defeated and slain by Klovis the Redeemer in the comic books.
C.S. Goto made some Ko'iron scribe a sort of magical ratskin messiah, but fuck him. It's just as autistic as it sounds, and if you're signing up to be offended, then it pretty much covers some pretty offensive Amerindian/Aboriginal stereotypes, too.
From outward appearances, Ratskin players get the short end of the stick when it comes down to available wargear. (at least initially, since you can always pick up a lascannon at the nearest outlaw trading post, though no guarantees your gangers will know how to use it!) A bare-bones gang comes with little other than a few low tech weapons like muskets and bows, even lasguns are rare special weapons (unless you play Dappakani) and you get no armour.
However, they do have access to some interesting stuff that kinda makes up for their inability to lock-and-load.
- Blindsnake Pouches: Though they are usable by any gang, they are made immediately available to every Ratskin ganger except the braves, and Ratskin Shamans can make more for free if they have the correct psychic power. Blindsnake Pouches are bags of toxic fungus soaked in albino blindsnake venom, the results of which can soak into the skin and activate a person's latent psychic potential. This doesn't provide them with psychic powers or anything, but does give them an intuitive sense of when they are being watched, kind of like a Spider-Sense, and allows them to dodge out of the way of unexpected attacks. This translates mechanically to an unmodifiable 4+ roll to negate overwatch shooting against them thanks to their sudden dodging and weaving.
- Blunderbuss/Scatter Gun: An extremely low tech version of a shotgun, amounting to little more than a metal tube packed with shrapnel and gunpowder. You go have a short range of up to 9", but gain a hit modifier of +3 at short range (up to 6") meaning your chances of missing are negligible.
- Coup Stick: Based on the Plains Indian tradition of "counting coup", where a warrior would prove his bravery by attempting to touch a dangerous foe (like a mounted cavalryman, or a Bear) with his bare hands or the wrong end of his weapon, and then escape with his balls still attached. Any Ratskin can take a "Coup Stick" into battle, which counts as a melee weapon and takes up a hand but does absolutely no damage at all. The benefits for doing this is that if your warrior ever takes an opponent out of action in close combat while holding this useless stick, he will have "proven his bravery" and will gain additional experience for doing so.
- Handbow: A crap pistol that can't be used in melee due to its cumbersome nature and the user cannot move and fire with the weapon as a new arrow needs to be placed manually. Although it has S4, which is better than auto/las weaponry it is still inferior to both as it has no positive to-hit modifiers and in fact any target with a save of 4+ or better increases its save to 2+ to represent the primitive nature of the shot and how poorly it performs against modern protections.
- Musket: Yet another low-tech weapon like the blunderbuss, except it fires single shot rather than a spray of pellets. Although it has the longest range of the basic Ratskin weapons, it's crap... inferior to virtually every weapon that other gangs get because it has no positive hit modifiers at any range bracket, as well as crap ammo rolls meaning it is likely to run out. Like the Handbow it also cannot be fired on the move, as the weapon needs to be reset after every shot.
- Spirit Staff/Totem: Essentially a low tech Force Weapon that will only function as such in the hands of a Totem Warrior because they are empowered by the "spirits of the hive". A one-handed "Totem" is about as good as a Power Sword (without the ability to Parry) but is essentially useless as loot for any other player if he captures your warrior and counts as a club. The two-handed "Staff" variant is superior to a two-handed power axe available to any other gang, with a high strength, increased save modifier and ability to do D3 damage with each hit it is one of the better weapons in the whole Necromunda setting.