なぜ僕の世界を誰も覚えていないのか？

Synopsis

"Why doesn't anybody remember the true world...!" The great war between the five races over the world ended when the Hero Sid led humanity to victory. A young soldier named Kai watched as that world was suddenly overwritten before his eyes. In the rewritten world, Sid never existed. Humanity had lost. There, dragons and demons ruled the world. Kai was forgotten by everyone around him. However, after encountering the mysterious girl Rinne, Kai decides to break this rewritten destiny. In a world without a hero, he inherits the hero's sword and martial arts and challenges the mighty enemy races in power.