The Chaser invokes a long-time trait in Slender Man stories, of humans that have been tapped to work for Slender Man or are otherwise driven insane and feral by his influence. Visually, The Chaser is very reminiscent of Marble Hornets' "Masky" and "Hoody" characters.
The Chaser is actually Kate, the friend of Lauren's who went missing. Kate herself is playable in Chapter 4 of Slender: The Arrival, "Flashback", from before she became The Chaser. Starting in patch version 1.3, a sixth chapter was added to the game, called "Genesis", in which Kate is playable. This Chapter is a remake of the original Slender: The Eight Pages (and also served as the beta for The Arrival), and as such, it is implied that Kate was the protagonist of The Eight Pages.