Naze Boku no Sekai wo Daremo Oboeteinai no ka?

Kai Sakura Vento knows history well: humanity won the great war because the hero Sid defeated the champions of angels, demons, undead, and beastfolk before sealing their armies away. As a soldier assigned to watch one of those seals, Kai lives in a world that treats the war as ancient history—until reality itself is suddenly “overwritten” before his eyes.

He awakens in another history where the enemy races won and humanity was nearly destroyed. Worse, nobody remembers that Kai ever existed. Even Jeanne, his friend who now leads a resistance force in this world, sees him as a stranger. The only person carrying memories of the correct world, Kai encounters a mysterious girl named Rinne and decides to restore the history that vanished, even if doing so means fighting whatever power erased him from it.