Soulless are people who have lost their souls.
It is like they are completely wiped of personality/etc. Their body decays around them as if they were extremely ill. They may follow orders, but they do not behave consciously, and must be told word for word what to do and helped throughout the process.
Tests done by the Coto government during the lead-up to their civil war discovered that minor trauma directly to the brain that would usually cause the death of a conscious person doesn’t seem to effect the functions of a soulless. It is known that this research lead the Coto government to learn the cause for people to become soulless and that the threat of turning soulless was used heavily to keep their populace under control, but that information was destroyed by Colla’colkort after the war ended to prevent the slave-trade from reemerging.
While they can be killed through injury, it has been discovered that their vital organs (eg, the heart and lungs) are no longer working. They cannot drown or suffocate, as they do not breathe, and cannot bleed out.
Unlike zombies they aren’t rotting and are not dangerous. And as they aren’t rotting, they die at a certain level of physical decay, although it seems to vary from person to person.
There are arguments of whether a soulless is actually “soulless”, or if it is a form of coma and the person inside is conscious, even if they can’t control their bodies or actions.