Slender: The Eight Pages
In the first game, Slender Man has no backstory and few characteristics. He is simply a tall, thin, and faceless interdimensional being that follows the player; becoming more and more aggressive as they continue to collect pages. He does show certain abilities, such as teleportation, distortion, and the ability to protrude tentacles from his back.
Slender: The Arrival
In Version 1.0, Slender Man is viewed simply to be a hallucination shared by Carl Ross and Kate following a traumatic event that happened in Oakside Park one night when the two aforementioned characters were kids. Slender Man's abilities are also enhanced as he now has advanced teleportation and distortion abilities, along with rare movement animations.
In Version 1.3, it is confirmed in the new level, "Genesis", that Kate was the young girl from the first game.
In the Steam Update, Slender Man's backstory is explained a bit more. In the level, "Homestead", Slender Man is referred to in collectibles by Frieda Matheson as a demon who was accidentally summoned during a ritual outside the chapel on the Matheson Farmstead. In the level "Memories", Slender Man is also revealed to abduct children, such as Charlie Matheson Jr..
- Slender Man is never referred to by name in-game, he is only referred to as "HIM", or "HE" by Kate in the level, "Escape". The name "Slender Man" is only used in the achievements and game files.
- In Homestead, you can find a charred ground area with a circle made from several rocks, located outside the church. This heavily implies that Slender Man was unintentionally summoned by the Matheson and Hayes families through a bonfire circle ritual, which is a common exercise in demonology.