The Stalker

Nothing is known about the Stalker.

They first appear in They Lurk Chapter 5: The Plan.

You are in an abandoned school and have to open a secret hole or something.

Lore Related
Nothing is known, but he may be Grim transformed. This is unlikely as the chalk boards in the area say that they reverted his transformation to get info.

AI Mechanics
The Stalker is always following the player but is invisible, if the player turns their back they will become visible and stand for a few seconds. In that time you have to look at him or he will kill you. You can actually lock him inside a room or lock him outside of a room if you're quick.