The Mercenaries are multiversal "Wardens" originating from the G branch of the Complex. At a young age they are selected and trained on GU4 to become the best at what they do; kill. Once they graduate training, they are put into a programme that regularly selects the very best to be Warden of a world.
The programme is called Temporal Rift Incident Protection, known as TRIP, and deals with the spreading tears forming in the fabric of reality that are caused by "undead". These so called undead are people who technically should have died, but did not. The souls have already departed for the underworld, but the bodies have carried on living, so therefore the souls have not been able to reach the underworld and are instead in some kind of waiting room, waiting for the bodies to die. In most cases the souls can still control the body and carry on living a normal life. In rare cases the soul can become detached and the body and soul lead separate lives.
Vampires are a good example. They are living, but they're soul is no longer there, as seen when they stand in front of a mirror.
These cases cause what are called rifts between the living world and the underworld. The two parts of ths same person will want to reconnect, which disrupts reality and makes the very fabric between these worlds tear.
The Mercenaries are tasked with hunting down these individuals and finishing the job before the rifts can become any worse.
On GU1, they are put through rigorous training and testing to see that they are fit for the process. Upon completion, they are "Immortalised"; they are injected with thousands of tiny nanobots, who repair damaged cells and tissue and renew anything that's been lost or is dying. They are programmed to do this continuously, but they can be switched off back at the control centre on GU1 when the Mercenary is deemed of no more use.