Sometimes people like this are content to be humble, to remain unknown. I think there's a quality in that. Like the story of the poor woman in the temple who only gives two coins, and Jesus notes that she has actually given more than the rich people who were giving piles of coins, but doing so from a position where piles of coins is meaningless. Humble actions can be meaningful. Not being aware of this man doesn't take away from his actions. In some ways it strengthens it. You can be sure that the people you read about on celebrity magazines everyday can generally be ignored. There are plenty of people in the world you will never ever hear about but are the most amazing people you could possibly encounter. This is but one example. A staggering example, of course, given the magnitude of his action. But I doubt he did it to be famous and have his name tattooed on people. He did it because he could.

I would suggest that the reason you have never heard about this is that the man was Japanese, and therefore an 'enemy'. Victors write history, and it doesn't serve the victors well to highlight the fact that no enemy is a monolithic block of pure evil. That's probably why many atrocities at the hands of the victors and the powerful have gone (and do go) unmentioned.