The Forgotten, The Damned and the Cursed. Sinners are human souls bound to Hell after their mortal life ends. Such a fate comes tied to their Sin, and these can be many and varied, as well as hugely important to their lives on Earth, or something so minuscule that they did not know what they did wrong in all honesty. However, that doesn't matter. A Sinner is little more than a soul given form, and then infused with demonic energy to create a new body, thus the once human is 'gone' and the consciousness takes a form as a demon entity in the afterlife.
Unlike natural born demons, Sinners generally have NO shared characteristics that define them as (previously) human, or even the same species, let alone race, ethnicity, country of origin and so on. By and large, Sinners' forms suffer a drastic change after death, and this change is defined by three things: Their Sin (like cannibalism, mass murder, etc), the way they die, and something they HATE.
A perfect example is the Radio Demon himself: Alastor.
Dead by a shot to the head, Alastor was also attacked by dogs before his death. Marked as a prey animal he woke up in Hell not only largely covered in scars, but also with deer antlers and hooves, ears that resemble a deer's and a few other characteristics that aren't confirmed and just mere rumors across Hell.
Let me offer you an example.
Cannibalism is a rather common Sin that lands you almost human looking however, partly because Cannibals are the closest to humans you can get for demons. Considering they are descendants from humans that lived in Hell, they very well should be! But things like singular, cyclopean eyes, the usual sharp fangs all demons (and almost all Sinners) possess, pale, bluish or grayish skin, as well as claws and a few other changes ARE still expected. Much like Alastor, who retained a largely human figure, and was a cannibal in life, there can be greater changes to mark a Sinner exactly as that.
There is only one constant in all Sinners: The Death Marks. Or, in a way, a particular scar that generally marks the area, or reason, for a Sinner's death. These tend to take the form of one or multiple red 'X', although they may not be visible on all Sinners, but they always exist, and in many cases are hidden, even if on plain sight, by the Sinner's power unless they let go and adopt a more demonic form as their self control and emotions flare up.
However, appearances aside, which can be as varied as imagination allows, probably more (As a Sinner can very well be a huge foot walking around!), there is one particularity that all Sinners share. Power.
All Sinners have the power of 'Immortality'; they are bound to Hell and cannot die unless some powerful magic, almost exclusively blessings from Heaven or those with the know how (and right magic) to produce them and imbue something (weapons or attacks) with holy power. Killing a Sinner any other way will bring them back somewhere in Hell, generally naked, in incredible pain (the worse off the Sinner was at the moment of death, the worse the pain), and weakened for some time. As such most Sinners do their best not to die in any way; one is extremely painful, the other is a permanent loss of their being as their Soul is taken away.
Immortality is not the only power though. All Sinners have increased strength, durability and are, in general, 'better' than humans, even if only a little bit. Some Sinners, either through actions in life, good or bad, or perhaps previous Deals with demons, if not simple luck and natural affinity to the energies of Hell, have much more prominent abilities. Such abilities can range from literal Super Hero (or Villian) powers, magical specialties and affinities (To a particular element or elements), or capabilities that have to do with things they already did in life, such as Alastor's radio based powers (And his affinity to Voodoo magic).
This all, if one ignores the Sinners' change in appearance, may make it look like a life of damnation is a prize, after all there are many other advantages. Sinners barely, if ever, get sick. Sinners can abuse drugs and alcohol with little to no negative repercussions (except possible addiction). They live forever and any bad time will, if one plays their cards right, turn into a good time at some point…
But Sinners have three negatives to focus on.
First: Sinners are all sterile. Death robs them of the ability to bring life. Only powerful magic, potions or other specific means may help a Sinner have children. Even then it is a costly endeavor as whatever was used needs to be reapplied during pregnancy and whenever reproduction may be attempted. And THEN, if the procedure is followed thoroughly, there is the likelihood of Heaven or other demons, hellborn or otherwise, catching wind of the process, which usually doesn't end well.
Second: Sinners are barred from going anywhere but the Pride Ring. There are, much like reproduction, ways to reach other Rings, or even the mortal world, but just like reproduction itself, these means are heavily monitored. Attempting to reach other levels of Hell, or Earth, by a Sinner is, if the proper means aren't used, met with a massive blow of force that can, in extremis, act as a blessing attack on the Sinner's soul. As such the yearly Extermination has a very easy way to counter the billions of souls in Hell whenever the angels come to hunt.
Third: Hellborn, by and large, dislike, if not loathe, or just plain hate, Sinners. Considering the VAST majority of hellborn are generally weaker than modern human souls once they have died, as most hellborn have very limited (if any) powers of their own and CAN die and be gone once they are killed (or their time comes in more natural ways), Hell has a long history of Sinners abusing the hellborn. As such it is not rare for Sinners, especially those that have messed with enough people and have a moment of weakness, to be attacked by the hellborn to finish them off with scavenged or repurposed blessed weapons. Even 'vanilla' killing Sinners is done on the regular just to inflict them pain if they are big enough assholes to mess with the hellborn population.
In the end Sinners are a varied group, and while many usually just mess around with fellow Sinners, including making Deals or making each other's life miserable (or, in rare cases, better), there are many that rile the place by screwing with the hellborn. They are a mass of people that come from all walks of life and can be either assholes, or usually turn into one after they spend enough time in Hell and have to survive what goes on daily.
There ARE good people among the Sinners. Just as there are bad ones. Just remember that Hell tends to bring the worst of everyone, and having powers, immortality and the ability to do what you want with little repercussion… Well, it tends to change people.
And change, as you may very well guess, is not always good.