I wonder if Hiro's zero score is hardcored in the game code, so it's impossible for him to raise it. That is, normally all the characters' score would be an integer variable, but since Hiro's only role is to hamper romance and be annoying before dying (giving the players the satisfaction of enjoying his death), then it would be better for the gameplay to hardcode it to zero. Assuming Hiro is ever present during fights in the game, it wouldn't do for him to accidentally get score points, after all, if the game plot and dialogue occasionally refers to him as a zero score idiot.