Kou has Species Identity Disorder, which made me suspect that the story has fantasy races IRL.
Supposedly, deep down he's a vampire like his mother, that's why he bit the female chilldhood friend when they were children.
Adrenaline of combat works as an outlet for his impulses so they let him play tons of games and even let him attend a fencing dojo, but they knew it wouldn't work forever, so they made the machine.
In the WN there's a flash forward, that months later he looked like the game avatar IRL.