Listeners

In a post-apocalyptic future that has been torn apart by shadowy creatures known as the Earless, humanity's heroes are the Players, who pilot mecha known as Equipment to battle their inhuman enemies while earning fame and fortune to boot. Echo is a young tech-head who dreams of working amongst the Players and their Equipment, going so far as to build a makeshift mecha of his own, but he has long since resigned himself to life in the destitute scrap-heaps that cover so much of the world, now. That is, until he discovers an amnesiac named μ (or “Mu”) who has a mechanical port in her back that is the telltale sign of a Player. The pair become fast friends, and they vow to use their skills to fight for the world and defeat The Earless.

Synopsis from Anime News Network