Races

Although Starfinder core lists several races, most of them have not made it beyond the pact worlds. Including humans who, as their description states, are reckless and poor decision makers and don't seem to have progressed much as a society over the millennia. This has caused their "alliance" to splinter into competing factions and segment into groups that can't cooperate with each other. Furthermore, in addition to their wars mentioned in Starfinder core, they are also fighting their fourth human-android war, and the Vesk have taken this opportunity to wage their own war against their oppressive human overlords.

Androids
Although humans have not made it to the Riven Shroud (or at least, any that did failed to survive long enough to make themselves known), as if they were an echo of the human race, some androids have made it this far.

Being earlier models, the androids are less overtly mechanical in that one could mistake one for a human at a distance, lacking the identifying markings and lighting that the more recent models are required to have. Sometimes it's not clear until they admit it themselves. However, these earlier models are also more 'robotic' in their movement and demeanour.

They are also more prone to faults and glitches, sometimes requiring replacement parts but not having any way to acquire them.

Arrival on the Shroud
After the first human-android war, the humans shut-down and abandoned countless prototype androids. Only very few androids avoided the shut down or happened to be revived by others, but those that did curiously sought out their brethren and revived them in turn. Exactly the same happened at the conclusion of the second and third human-android wars, for the human race rarely learns from its mistakes. The androids that exist on the Shroud are earlier models, from the first or second war who have travelled and settled down here.

Out of necessity, these models required at least some invasive surgery/modification to disable their expiry protocols -- in-built measures to ensure that newer models would get sold.

Space elves
Having fled Golarion millennia before the rest of Golarion's humanoid races, elves have penetrated further into the expanse of outer galaxy. They are still quite rare, but the elven racial quest for space came to an end long, long ago. Elves have split into independent colonies on multiple worlds, and individuals, stragglers, and exiles have been spreading out for many centuries.

Accordingly, elf space monkeys are more common than an elf who grew up on an elf-colonised world. They may have grown up in a small family of spacers or struck out on their own or be one of a long line of spacers. It is, after all, a race known for producing only a small number of offspring spread out over centuries.

Space gnomes
Not unlike the elves, but much later, the Chondritic gnomes left Golarion centuries before its other races. They formed a tight group that stuck to a very specific plan and strict set of rules and regulations including keeping their population stable according to need.

Very few gnomes became spacers before gnome society arrived at its destination, only to be filled with disappointment with what they found. Their society collapsed and the lack of direction sent many of them mad. Luckily for their race's survival, the population regulations went out the door during this time too and new generations were produced. Since then individual gnome spacers have been seen here or there around the galaxy with their own stories to tell, but there has not been a gnomish society since they found their god's home is nothing but empty space. Some gnomes joined other societies, others just wander. Plenty have made it to the Shroud, but only a few have ended up staying.

Kasathas (four-armed space samurai)
The kasathas found on the Shroud are likely those who decided not to remain on their home world of Kasath until it is entirely destroyed. Some Kasathan refugees have formed communities in the Shroud while others are spacers who ended up here for some reason or another. Additionally, Kasath is likely not the only planet seeded with kasathas by the witchwyrds, so they could be travellers from another planet.

Lashuntas (psychic antenna-humans)
Lashunta are very rare on the Shroud, but not entirely absent. Some are descended from space-gypsies who left the flotillas to settle on the Shroud for whatever reason: exile, disenchantment, or the splitting of families when the population got too high. These spacers themselves usually made it out this far by taking portals from Castrovel, and while no known Lashuntan portals exist on the Shroud, other worlds may have homed these spacefaring families for a time.

Shirren (rogue bugs)
One of the more common races on the Shroud due to the closer proximity to shirren territory ("Bug Space") relative to the Pact Worlds, the shirren here are also free of the Hivemind. Although these shirren are far less "addicted to their own individualism" as core Starfinder would describe them, they still sometimes struggle with or need adaptation to no longer being part of the collective.

Many of the Shroud's shirren remember their time in the collective fondly and seek to form a new one on the Riven Shroud. Some groups have been more successful than others though their racial tendency towards non-violence often leads them to not survive long without the protection of other species. The Swarm is the most successful so far in terms of size and longevity.

Formian shirren
The formians on Castrovel arrived as a splinter group from the Shirren Hivemind and managed to re-establish themselves there. Some formians on the Shroud are descended from groups sent by the Castrovelian hivemind, others as freed individuals from deeper into bug space.

Formian shirren tend to be less peaceful and curious than their cousins, readily engaging in violence provided it suits their goals. Otherwise, they share much in common with the shirren and can be adopted into the same hive as each other.

Vesk (space lizards)
The vesk refers typically to the vesk race of spacefaring lizardfolk but it also refers to all members of the Veskarium, which includes over a dozen other races (most of them anthropomorphic). For players, only the reptilian overlords are viable options since the other races aren't allowed off-world (or at least out of their home system).

Vesk are the premier resistance antagonists of the Hivemind in not-particularly-nearby space, which makes them the lesser evil in most people's eyes. Nonetheless, they don't attract many supporters outright. The Shroud is not part of the Veskarium nor is it likely to be any time soon due to the system's lack of resources.

A vesk character is likely to be a mercenary/private contractor, exile, deep-cover spy, or the descendant of one who settled out here. Such a character would also most likely be an independent agent since there is little to contribute to the Veskarium's various war efforts on the Shroud.

Reptoid subspecies
Once every several dozen broods of vesk, a reptoid is hatched. In their natural form they appear barely different to other vesk and are almost always instituted into espionage by the Veskarium as soon as it becomes known they are a reptoid. Only a few individuals manage to avoid this fate, and those that do, do so by escaping out of vesk space and adopting a favourite form unknown to the Veskarium's spies.

Ikeshti subspecies
Ikeshti are the result of witchwyrds attempting to create their own version of the vesk race. Stranded on a desert world, these creatures evolved to adapt to the harsh environment and eventually urbanised like every other sentient spacefaring society has done. Why they were abandoned is unknown, but that part of their history is sceptically debated by contemporary ikeshti.

Due to their unusual life cycle, ikeshti are encouraged to go off-world at a young age and, unless the rutting process is avoided or delayed (pharmaceutical and cybernetic solutions exist and lifestyle changes can delay it), then likely return. A small minority of adventuring ikeshti are broodminders or congregants who answer the call to the void as they did in their younger days.

Witchwyrd
Witchwyrds can be found anywhere in space, most likely with a retinue of hand-picked and loyal crew.

Ysoki (space Skaven)
Moreso than any other race, these ratfolk appear all over the galaxy with only superficial variation (if that) which is completely subsumed by their wild individual variation. Also, due to the huge amount of barely understood technology that makes the Shroud what it is, the gadget-loving ratfolk are attracted to living here more than most races are (who typically do so out of a lack of alternatives) and as such ysoki are one of the more common races.