Slenderman is the engimatic antagonist of TJAProjects, and appears in several episodes of the series. It has not explicitly appeared in any of the associated follow up vlogs or series. Of note, the existence of the fictional Slender Man Mythos is at least acknowledged within the series. References to other Slender Mythos works such as Marble Hornets and Everyman HYBRID are made by TJ and Amy during the course of the series. Indeed, some element of the Slender Man Mythos is the inspiration for the film project which TJ and Amy are filming as of the first episode of the series (it is unknown whether this was the original SomethingAwful imageboard thread, stories, blogs, or one of the aforementioned vlogs).
The Slender Man is typically seen as a tall humanoid of slim build, invariably seen with a black suit and white dress shirt. The proportions of The Slender Man can vary with appearance, but generally suggest that the arms and legs of the entity are somewhat longer than thinner than a normal human's. Most memorably, the Slender Man lacks any facial features or hair on its head, resembling a bald, faceless caucasian humanoid. In some instances, the hands of the Slender Man can be seen, which are enlongated and either do not have normal skin, or has very dark (bordering on grey-black) skin. As seen in several episodes, the physical touch of the Slender Man can greviously injure humans - leaving large second-to-third degree burns across unprotected skin. These wounds normally resemble hand and finger prints, although proportionately large given the entity's size.
The Slender Man also has some unknown, unexplained detrimental effect on recording equipment, photography, and other electronic equipment. The effect is inconsistent, but normally manifests as audio and video distortion while it is in the vicinity of the machinery in question. While the Slender Man is primarily seen at night, it's activity is not restricted to nocturnal hours.