Possessing a Girl, Picking Up Memories in Another World

“The portal to another world has opened, bro, but nobody expected it to turn out like this.”

“Disaster? Extinction? Invasion?”

“People's fears about what might happen after the otherworldly gate opened faded away a few years later. But some of those things really did happen, my friend.”

The amnesiac protagonist becomes a biomechanical being and regains his lost memories in another world.