The Shadow People are actually the Corrupted Souls.
They're both featureless pitch-dark vaguely humanoid figures with only two glowing white spots for eyes and very malicious intents.The games exist in their own universe as an autobiographical work.
The protagonist canonically chose to take over the traveler's body in order to show the world what he saw. He programmed in the opportunity not to do so partly to try and ease his own regret.Amy's brother is the protagonist from the first three games.
The fourth game will feature a young woman looking for her dream-obsessed brother who supposedly died a while ago. This will probably be either confirmed or refuted when the game comes out. If it is true, then it gives a name (Thomas) to the previously-unnamed player character.