In the final years of the Afterwar, it seemed as though the UPD was running out of manpower - with the maralyn's male-selective viruses constantly evolving and wiping out human soldiers, and the dmargo's various superior TA squadrons demolishing heavy armor and infantry alike, the general moral of the soldiers plummeted as far as the bodycount went high.
Arsaw, the UPD's main weapon manufacturer after Runlar Corporation disbanded, came up with a plan to start recycling the fallen soldiers. With their limited knowledge of cybernetics, they managed to effectively introduce a primitive system of electric impulses that could move a corpse's limbs, even after death. As long as the body hadn't suffered extreme trauma to the brain or spinal chord (i.e., missing them), Arsaw could combine a rudimentary AI program with the host's original brain, in the hopes to restore the body back into some form of fighting shape.
It half-worked. The Recreo were monstrosities at best, but still more useful than a cold, rotting body. Able to wield their custom Arsaw FS-30 sub-machine guns grafted onto their arms and throw grenades with relative accuracy, a Recro is not able to follow through with complex motor functions or commands. They proved most useful for standing guard, acting as mobile turrets, or as "shock" value against dmargo Dusk Legion forces, who were unable to fathom or unwilling to believe what exactly they were fighting.
Recreo aren't fully aware, and Arsaw was fairly confident in the fact that a Recreo did not retain any former memory of their past self. Gas masks were secured onto their faces and heads shaved, mostly to protect the individual's identity. It wouldn't be beneficial to suddenly be fighting alongside your best friend, who just days before was killed in action - now emaciated with most organs removed to make for a smaller target, face twisted in death, shambling and uttering random phrases. A large battery / pump backpack is also secured onto a Recreo's back, pumping their body bull of preservatives and sometimes nutrient cocktails to prevent decay.
The UPD marines often refer to Recro as "Fiths." It came from their uncanny ability to remember and shriek the phrase, "fire in the hole," in their raspy, static filled and muffled voices when throwing their grenades. Some argue it could stand for "fucked in the head," as they occasionally mutter and mumble random, nonsensical phrases as portions of their brain are activated. "Black Pants" is another common term, reffereing to the single article of clothing given to Recreo in order to "humanize" them.