"[In the Tokyo Game Show demo] there were some upgrades provided to Raiden, but not too many. We wanted to have the players and fans enjoy Raiden not just as the character you start out with but with the character and cyborg powers that you gain throughout the game," Korekado said.
"The doctor is a key person in the game," he said. "Throughout the game, if you cut enemies' left hands and take them back to the doctor, he'll buy them off you. What happens is you use those as collectibles, as well as items to trade."
Korekado explained that the doctor character in Metal Gear is a mad scientist who is constantly after information about cyborgs and how they interact, so when Raiden brings him his victims' left hands, he rewards Raiden with upgrades both to his cyborg parts and his weapons.