“What do they want?” - Captain Duhane
“To make more of them and less of us.” - Turin
The Hullen have existed for thousands of years and across species. 
Hullen look like normal human beings, however they have a number of superhuman abilities that set them apart. Those who survive the bonding process have their strength, reflexes and endurance enhanced to extreme levels, as well as gaining the ability to rapidly heal from all injuries with the exception of sudden brain death. Their aging process effectively ceases as well.
The process also damages the individual's anterior insular cortex and hypothalamus, suppressing the capacity for empathy and bonding. Because of this, they maintain training camps around the galaxy, using human slaves to teach Hullen how to socialize and act "human".
Hullen Hive Mind
Creatures bonded with the neuroparasite develop a hive mind within their species. Mossipedes of the Badlands on Westerley are an example. The Hullen have a hive mind that allows them to access memories from all of their kind, as well as communicate instantaneously across interstellar distances. Due to an imperfect bond, the original humans that bonded with the neuroparasite, such as Khlyen, tended to retain a greater degree of autonomy, however Aneela and her allies conducted experiments to strengthen the bond in order to suppress autonomy and control the newer Hullen. These experiments formed the basis of development for the Red 17 process.
Hullen in the Quad
"Centuries ago, my people offered the ancestors of the Nine a deal, eventual assimilation or immediate annihilation. Their descendants have had ten generations to suck the wealth out of the Quad and honor the deal. What you're seeing in Old Town right now is the beginning of that handover. A silent invasion has begun."
Since then the Hullen have expanded their numbers and influence, and under Aneela's leadership will be everywhere eventually. They have infiltrated the RAC and The Company. The sole reason The Quad is of interest to the Hullen is its plasma. That is why Khlyen, Dutch, Johnny, and D'avin determine a way to destroy the plasma, to take away the Hullen's need for The Quad and to stop their ability to make new soldiers.