Naze Boku no Sekai wo Daremo Oboeteinai no ka?

Synopsis:

"Why doesn't anyone recall the real world?" The great race of five tribes that were fighting for hegemony on the ground came to an end when humanity, led by the hero Sid, won. However, in front of a young boy named Kai, that world was abruptly "overwritten." Kai saw the scene in the rewritten world where humans lost the five tribal wars because Sid, the hero, was missing. Kai is a forgotten existence among humans, and the ground here is dominated by dragons and demons. In any case, subsequent to experiencing the secretive young lady Rinne, Kai chooses to break this revamped predetermination. He inherits the hero's sword and martial art in a world devoid of heroes to confront the dominant enemy races.