Why Nobody Remembers My World?

The story is set in a world in which humanity triumphed over other races in a war for hegemony. However, this is rewritten with a new reality where humanity lost the war of the five races, and its hero Sid is nowhere to be found. Only a young boy named Kai is aware of this change, and other humans seem to have forgotten his existence. After meeting the mysterious Rinne, Kai resolves to go against humanity's altered fate.