WMG / Slender

Kate's mother made the original notes
Kate and CR went ghost hunting near her house. Her mother died of unknown causes. Notes were scattered all over a park near the house.

Slenderman is the Doctor

The Player Character is the one leaving the pages around.

The new ending of the game shows Slender Man taking the victim regardless of collecting all eight pages and the screen goes static and then dark, like before. However, now the victim will come to and it'll be daylight out, but they're still trapped in the woods. The character is caught in an endless time loop and is the one leaving behind the pages, him/herself. This also explains why the pages randomly spawn.
  • The new Daylight mode certainly helps to reinforce this.

Slenderman isn't after the Player Character; it's after You!

In earlier versions of the game, the window would close if you were caught by Slenderman. If Slenderman appears in the character's vision, then the screen starts to become fuzzy. That's not the character's vision blurring It's your computer. Finally, if you are caught, the screen goes black and Slenderman appears on the screen looking directly at you!

The Player Character is the protagonist from the song, Slendy's Watching Me

The new ending shows that the Slender Man doesn't physically harm the Player Character if she gets all eight pages, and the above theory hints that she's locked in a never-ending cycle. Why would the Slender Man do such a thing to her? Because from his point of view, he's simply playing a game with his little friend! As the song ends, it says that Slendy is taking her to a place where "[their] fun will never end." This is indeed a game that never ends, and like in the song, he cheats at it. Being a child would also be a good explanation for why the Player Character has pretty poor stamina. However, given the appearance of the pages, the protagonist probably no longer thinks her "newest best friend" is as benign as she originally thought.

The truck near the shed is the reason that Slendy wants 20 Dollaz, and he's chasing the player because he wanted to ask for gas money.
The truck is his and the tanker was carrying gas. When the original owner of the truck drove into the woods Slender Man did... whatever it is he does. He then decided to go for a drive, until the tank went empty. Then, a tanker carrying fuel drove into the enclosed part of the woods where the game takes place, and that person got Slendered too. Now he needs money to fill up the truck so he can go driving again, since he abducting people has so far only been a temporary fix. And he's chasing the player character because he thinks she has the money and just wants to borrow 20 dollars. Of course, the player just sees "tall creepy skinny man with tentacles that makes my camera freak out", and decides to just book it. Slendy doesn't take that well, so keeps chasing her.

In future versions
You'll encounter Masky or Hoody.
  • Don't forget about Trendy!
  • Or the Rake...on second thought, lets hope not.
  • It's doubtful we'll ever encounter the Hoody or the Masky in this game series...but the trailer for The Arrival has shown the Player Character being chased by a hooded masked man.

The next game will be expanded into the Creepypasta universe.
And even crossover with Marble Hornets and Everyman Hybrid.

The Slender Man can't see; He looks through your eyes.
That's why he has a harder time following you if you don't look at him. You're easier to go to when he knows where you are in relation to him. Also, he can sense the pages, which is why you're easier to follow the more pages you pick up. Another reason for this is that you've spent more time in the woods, meaning his mental map is getting more accurate as you see more.

The random cross shaped brick wall was constructed out of the bricks that everyone shat.
Because it makes more sense than anything else. A common theory is that the Slender Man feeds on fear. Maybe he's collecting the bricks shat by his victims to construct a wall. And stuff.

The pages are a Secret Test of Character by Slenderman
Since the gameplay involves sneaking around to find pages, evading the pursuit of Slendy all the way. If he catches you before you find all the pages, he... well, you can probably guess. BUT, if you find all 8 Pages before he catches you, he deems you worthy enough to serve him, and instead of devouring you, makes you serve him as a Proxy! Um... congratulations?

Slenderman is just defending his home from a robber (aka you).

The fence and the eight pages are what he uses to mark his territory. From his perspective, you climbing over the fence is rude enough, but taking the pages is downright criminal, especially when one of them outright spells out "Leave me alone".