People who were summoned or reincarnated into the world of Death March kara Hajimaru Isekai Kyusoukyoku from another world.

Summoned means the person was forcibly abducted from its previous world to the world of Death March.

Reincarnated means the person had died in its previous world and reborn in the world of Death March.

It is still unknown how Satou crossed over as he neither reincarnated or summoned.