Is Mr. Matherson or Charlie Matherson Jr the Slenderman in Slender: The Arrival?
Solution 1:
Playing through the game, it is revealed that
Charlie is actually the blue creature at the end of the game,
he can't be slender. It also is less likely to be his dad because he says in a note that Charlie is gone but he is looking for him. Considering Slender is primarily focused on kids, this makes perfect sense.