The Human Race
Many creatures, beings and races started arriving on Earth even before mankind noticed Bruna 8 in the universe, but while these extraterrestrial beings were relegated to the lower rungs of society on Earth, the discovery of Bruna 8 gave them a new purpose and profession. Since most of the population still consisted of normal human males and females, that's who most factions sent up to Bruna 8 but most served as cannon fodder only. Funnily enough, it was the other human variants that were traditionally looked down upon that became very valuable due to their unique abilities which made them suitable for combat.
After the collapse of nations, the Yakuza clan once again emerged from Japan. They started off becoming the de facto leaders in the region of Japan but soon their tentacles spread across the globe, turning them into a formidable faction to be reckoned with. Some Yakuza members voluntarily went to Bruna 8 after learning of the exciting opportunities up there, others were ordered by the Yakuza faction leaders to go up there in order to win the tournaments for their clan.
Zombies and their existence has always been a taboo topic on Earth ever since late 20th century, no one really knows where or how the zombies came about but they have always been there before the contraction event. There were many attempts by both humans and zombies to reach a truce and eventually the zombies were accepted into society, but not for the reasons they initially wanted. In the end, they were accepted into society for the sole purpose of being used as warriors on Bruna 8, their natural thirst for human blood and hyper-aggressiveness makes them the ideal warriors against other human variants in the arena.
The 'Mutant Apocalypse' started harmlessly, with newborns born with defects soon after the contraction event but unlike what people expected, these so-called 'mutants' were harmless. They were friendly and non-aggressive, most people just saw them as normal human beings with enhanced strength, speed and stamina. As new mutants were born, they soon made up a significant percentage of the population. It was their physical superiority that made them ideal candidates as warriors on Bruna 8.
They are the hippies of the 2040s. Different and misunderstood in the same way their ancestors were. Adorers and admirers of substances make you see things left Earth searching for newer and more unique drugs to try. Desperate to reach Bruna 8 for the BYOPills, they devised numerous schemes from falsification of ID cards, plastic surgeries and identity theft to get onto Bruna 8. Whatever they ingested on Bruna 8 gave them more than the trip they were looking for, it gave them superpowers that made them more powerful in the arena than they could have ever wished for.
Cyborgs were originally normal human beings on earth, they became handicapped as a result of the skirmishes that broke out after the contraction event. Luckily the technology in the 2040s allowed them to stitch themselves up quickly and very affordably. Once the recruitment for Bruna 8 warriors was announced, some even began to equip themselves with parts that boosted their combat abilities to increase their attractiveness to faction leaders.
Ah.. the Gold humans, these are the faction leaders; they are the wealthy elite who now controlled the new world. They were normal humans too with normal human skin before the contraction event but underwent a process to permanently add a layer of gold on top of their skin since the contraction as scientists advised that the only naturally occurring mineral on Earth which could protect them from the radiation and harmful rays was gold. The price of gold immediately 100x'ed overnight, and only the ultra-wealthy could afford to give themselves this new protection.