This is Ruen. He lives in a post-apocalyptic world, with only a small band of fellow survivors. This apocalypse happened when aliens tried to consume earth's sun, to make sun-crystals to give them life energy. They wiped out most of the world when they half destroyed the sun, because no human can live with a faded, cold, red sun. The only humans that survived were those who had robotic body-parts, because their prosthetics operate on sun-crystals, which gives them life just like the aliens. Ruen is a survivor. His right arm, you can see, is mechanical below the elbow. His sword is handy for fighting the sun-deprived aliens that got stranded on earth.