The Slender Man is a creature or being with various nebulously defined characteristics and abilities. Slender Man generally appears (in modern times) as a tall man in a black or grey suit, red or black tie, and white shirt, with no eyes, mouth, or clearly defined facial features. It has no hair, and generally has normal-looking bare hands. Slenderman typically is depicted in imagery and literature as between 6 and 15 feet tall, depending on the situation, and in video as around 6-7 feet tall. Because of its inexact nature, and differences between series, no one has yet determined what it is. Currently, the two leading theories as to what the Slenderman may be are the Tulpa Effect and Quantum Theory.

Slenderman's original abilities included invisibility to select people, adult humans, or anyone without use of a camera, the ability to change height and body shape, the ability to shape its arms into tentacles, and the ability to sprout tentacle-like appendages from its back and shoulders to be used as additional arms or legs (or simply to instill fear into its prey). Slender Man also had connections to children, with many of the original stories revolving around disappearing children or featured Slender Man being sighted in photos with children.

EMH, TT, ML, and DH all contain some sort of video and/or audio distortion, indicating that Slenderman has some sort of effect on modern digital instruments. This does not appear in any of the original accounts- however these mostly make use of photographs and video of primarily lower-tech varieties. EMH Has applied this distortion to be the effect of "Sigma Radiation", a radiation given off by Slender Man and The Rake. Sigma Radiation is also contagious, as it entered Evan's blood-stream when his arm is brutally maimed by The Rake.

EMH and ML also make use of "Slenderwalking", a term created to describe Slenderman's apparent teleportation, appearing, and disappearing at will. EMH independently indicates the use of selective invisibility to groups or individuals through head tilting, and also spacetime warping/teleportation between locations. Similar abilities were seen in early Slenderman accounts. EMH also hints at the affecting of mental health or control of it.

Slenderman has no exactly defined or specified history, however contributors have placed early sightings of Slenderman-like beings known as Der Ritter and Der Großmann in early 1600s German and before, where it took the dress of a knight or royal figure. Germanic fairy tales and mythology also makes use of the creatures to be used as cautionary tales for children.

Photographs from the early 1900s were the next confirmed reports, where imagery of Slenderman can be found in classical black and white and sepia imagery. Reports from this time indicate sightings in America, the UK, and Russia, as well as reports of child disappearances.

By the mid-1900s, some rare run-ins occured in the warzones in Germany, apparently Slenderman's native land. Soldiers were the primary targets here. In America and Canada, reports of missing skiers and children appeared, coming from forested areas of the nations. After a spree of building burnings from unknown causes, several deaths and a few surviving witnesses, it could be gathered that the Slenderman had the following traits: