i've always been speculating that D and by extension the rest of the footsoldiers are the collective of the unborn/newborn dragon priest since yumeko was talking to a pregnant dragon priestess in a flashback in ch 105
I've had this speculation that maybe all of the Footsoldiers were once an executive. That it might be the Rat executive who gave up their being, or was on the verge of death, to become numbers who could continue the fight long after it was gone.