The Smiling Man

The Smiling man first appears in It Lurks Chapter 1, you will hear a knock on the door and be told to open the door for your father but you're supposed to not open the door as it is The Smiling Man mimicking your fathers voice, he was evacuated already and so was Grim which is why he isn't there when you wake up. If you look out the window and don't open the door you can see The smiling Man running past. This doesn't mean anything other than it is safe but the game says when it is. This happens shortly after the emergency broadcast. The Smiling Man also has an appearance in They Lurk Chapter 3, he is very sensitive to light which is why you have to follow Chomper. He will follow behind the light and if you stop he will kill you. At the end of Chapter 3 when you pull the lever The Smiling Man appears behind you and says "Hello, human", the name of the voice is just "Voice" while some sounds play. Their next appearance in chapter 4 of They Lurk, he is just a little "scare" you could say, he peeks over from the corner and makes a screeching noise as he runs back. If you decide to follow him into the darkness instead of pushing the button he will kill you.

Then in They Lurk Chapter 5, when you find the first lever the lights will shut off and you are required to hide inside of a locker to live, unless you just get out of his sight. Which I do since his current face is kinda funny.

Lore Related
Nothing is known yet.

AI Mechanics
The Smiling Mans AI just stays away from light, he actually never tries to detect players if they're in the dark, he just is coded to stay away from Chompers lantern. Most of it is just intelligent trickery, like that big room in the metal labyrinth with a few lights on the corner to try and appear as you have to avoid the dark. But you can just afk there and nothing happens!