The unknown species, monster, is a parasite that needs a body to survive. The parasite's only option was to latch onto a teen boy as he walked through the woods, stealing his body and using it to preserve it's life. It manages to have all of Tae's memories and create a personality to mimic how the real Tae would act. It blends into society, acting as though nothing is wrong with its use of a poor teen boy's corpse. It has Tae's memories but doesn't have experience itself, like a newborn.