Kate the Chaser (last name believed to be Milens or Hayes) is the secondary antagonist in Slender: The Arrival, and the unseen protagonist in Slender: The Eight Pages.
Slender: The Eight Pages
Kate is the (albeit unseen) player controlled character in Slender: 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.
In Slender: The Arrival
Kate has multiple appearances in the game. Between The Eight Pages and The Arrival, she appears to be in her late teens to early 20's.
Before the setting of the game, Kate and her friend, CR, always went out "ghost hunting" in the woods. They would hallucinate things and start running back to Kate's house. A few years later, Kate was found with a broken video camera, flashlight and a stash of weird drawings. She was soon taken to a psychiatric ward, and had tests done on her. She tried to explain to the doctors, but they would not believe her. A couple of years after the traumatizing experience, Kate was considered stable and sent back to her home. She spent her time with her mother, who soon passed away due to unknown reasons. Kate soon began seeing Slender Man again, and started to draw him more and more often.
Kate was also close friends with Lauren during their childhood, and the two of them would always play on the swing set outside her house. This is what prompts Lauren into investigating Kate's disappearance in the first place, which ultimately places her in mortal danger as the Slender Man starts attempting to kill her.
A video in Chapter 4: Flashback, shows Kate drawing Slender Man, and muttering that "He's here!" She tried her best to barricade the doors, but she soon found Slender Man already inside the house, and desperately tried to escape to her room. Slender Man was right behind her, and as soon as she saw him appear in her room she is either thrown out of her closed window by Slender Man, or she leaps through it as a last-ditch effort to escape. The tape ends with Lauren baffled.
In the exact day that the game takes place, Kate runs to the woods after fleeing from the video, but in vain as she was caught by Slender Man. She shrieked as loud as she could, making Lauren hear her from a distance. Lauren races to where the scream was, only to find nothing and be stranded near Oakside Park, where Lauren is then captured and teleported to various locations.
Chapter 6: Genesis (added in version 1.3) is a flashback, and confirms that Kate is the person in the original Slender: The Eight Pages game.
Kate is soon found in the Kullman mining shafts chasing Lauren. At this point Kate has become a proxy to Slender Man known as "the Chaser". It is unknown what happened to her, but she now seems feral and wears a mask and has a black substance around her eyes and mouth with very pale skin, possibly due to residing in the mines. Her hoodie and hands are also heavily stained with blood; it is unknown whose blood it is. It appears that her eyes are very sensitive to light. She gets dazed when Lauren concentrates her flashlight's beam towards her face. She also appears to snarl and growl. She keeps chasing Lauren until Lauren finally gets on the elevator and escapes.
In Chapter 5: The Arrival, after Lauren gets knocked out by a proxified Charlie Matheson Junior. Lauren wakes up in a room with him. After collecting a book, Charlie disappears, and the crying of Kate is heard. Lauren finds a normal Kate (unmasked and unhooded as well looking badly burned) crying in a corner. Lauren approaches Kate, only for her to turn back into the Chaser and presumably kill Lauren. The last shot of the game shows Lauren's limp body being dragged away by either Charlie, Slender Man, or Kate.
- She is very sensitive to light. (In Slender The Arrival, right click to blind your foe.)
- She was created by Blue Isle Studios for the video game Slender: the Arrival. Here's a link to a walkthrough of the game: 
- Kate's last name is either Milens or Hayes. Her email is "firstname.lastname@example.org", that could mean Kate Milens. Also her mother's name is Beth Hayes. However, it could also mean that Beth is possibly her stepmother.