The Chaser

"He's here."

- Kate's only quote upon encountering the Slender Man for a second time

Kate (last name: unknown, assumed to be Hayes or Milens) is the main protagonist of the original Slender game and the secondary antagonist of its official sequel, Slender: The Arrival, acting as the main antagonist of the Into The Abyss chapter, she serves as a brainwashed proxy who relentlessly chases the player before pouncing on them and ripping their pinned body apart.

Slender
Kate is the (albeit unseen) player controlled character inSlender: The Eight Pages. According to Slender: The Arrival Kate and her friend CR had a habit of going into the forest frequently as children, flashlight in hand (thus why the player starts with the flashlight in the beginning of the game). They would spook themselves on purpose for fun. According to one of the notes, at some point he got lost in the dark, hearing Kate screaming. He eventually got lost and wound up at Kate's house, where she was sitting in the living room. CR tried to get her attention, but failed to do so, and went home.

From Kate's perspective, she was chased around the forest by Slender Man as she found notes on various landmarks.

When Kate is reported missing, her friend, Lauren, arrives on the sight, looking for her former confidant. Lauren is hunted by the Slender Man as well before stumbling upon an abandoned mine shaft. It is in the shaft where Lauren discovers that Kate has been brainwashed to become the Slender Man's proxy. As such, the mindless villainesses hunts the player sided with her master. Lauren eventually powers the elevator and leaves Kate trapped in the mines while Slender Man teleported out and resumed his hunt, escapes into a burning forest and is knocked unconscious by another proxy, Charlie Mathewson Jr who was proxified long before Kate. When she wakes up she finds herself trapped in the Mathewson homestead which was burned down by either Charlie or Slender Man and resulted in Charles Mathewson Sr's demise, When Lauren finds Cr's last letter to Kate, Charlie (who was guarding Lauren) disappeared and Lauren heard Kate's cries Lauren traces the source to Kate who's will and mind have been broken by the Slender Man, Lauren tries a desperate attempt to reach out to her friend by Kate mysteriously and quickly donned the bloodied white jumper and mask and leaped and (presumably) brutally murders Lauren before dragging her bloody corpse away for unknown purposes, (due to a sequel game not existing yet it is unknown whether Kate and Lauren are still alive).

Trivia

 * Kate was originally mistaken to be Jeff the Killer. This is because the Slender Man and Jeff are usually compared to each other and Jeff's attire is similar to Kate's (white, blood stained hoodie, black pants, pale face).
 * Kate is also very similar to the Hunter, a villainous mutant/zombie from Left 4 Dead. Both are antagonists who wear hoodies and attack their enemies by pouncing on them before ripping them to pieces.