Skin-stealers ingest scraps of skin from corpses, typically removed from the chest above the heart. This, along with some catalyzing component, allows them to wear the victim's skin as their own for a prolonged period. It seems the ingested skin provides limited memories from the victim, which allows for such thorough infiltration. A few skin-stealers have slipped up and given themselves away, but we have no idea how many are in our ranks.