It's been a long time since I read it, but I've been looking for a manga that I had really enjoyed and of course forgot about. While I don't remember much, I do recall that the setting was fairly generic now, Hero going against the country that hated him, goes to join the Demon Army. However he had to hide his face because the demon long won't let him join. He ends up gaining the trust of the Heavenly Kings while also testing them to make them better leaders. Also, I believe that in the later chapters it's revealed that he is actually incredibly old, and has been protecting humans since demons started attacking them.