@blkchainicarus It could be that venom was experimented on with those secondary parasites and the adrenaline rush he gets as he's always in pain is what connects him to the 3rd boy and the Phantom Pain-esque cold shiver down your spine when you're being watched is what explains reflex mode!
@blkchainicarus Also Volgin has the Fury's parasites while Quiet and Code Talker have the End's parasites. I believe in the parasite tapes Code Talker identifies two parasites that belong to the Pain which is one for insect control and another that give an adrenaline rush in response to pain.