He isn't the only one.
Everyone has amnesia. None of them remember who they were before their arrival. Considering the danger they now face, not being able to remember their past might be for the best.
This world is a mysterious winter planet, and surrounding them is a place they call the Moat, where something deadly lurks. No one knows for sure what it is, but anyone who goes exploring goes missing.
But they must find a way off this planet, so the survivors are gearing up for another push into the Moat. It's the only way.
As the newest rookie, he's going too. At least they gave him plenty of weapons to choose from. He might not remember his past, but somehow he knows he's an expert marksman.