Slender Man, also referred to as the Keeper and the Administrator by the Collective and as Mr. Slim by Mary Asher, is the main antagonist of TribeTwelve. It is important to note that the name "Slender Man" has never been said or used in the series canon, possibly to avoid litigious action. Appearing in many videos throughout the series, it is best described as an abnormally tall figure in a black suit with a pale, featureless face and, occasionally, tentacles sprouting from its back. It is unknown what the Slender Man wants, but he appears to be following Noah and, before that, Milo and his mother. Initially, it can be inferred from the submission videos that Slender Man had been stalking Milo since at least May of 2008.

After Milo's death, Noah uploaded the contents of the tape from Milo's visit to Youtube, resulting in his discovery of the Slender Man. The Slender Man is represented by what has become known as the Operator symbol, an O with an X through it, meant to represent the Slender Man's lack of a face. The Slender Man is the leader of a group known as the Collective, a group of powerful beings that exist outside of the normal laws of time and space. It can be inferred that the Slender Man possesses god-like powers and omnipotency, some of which it bestows upon the Collective.

According to the video My Grandfather Karl, a similar creature known as "Der Großmann", or "The Great/Tall Man", was the subject of a legend in Germany. The creature would kidnap children who ran away into the Black Forest. Karl Maxwell encountered it when it attacked a Nazi soldier. According to Karl, the same creature appears in Noah's videos.

Slender Man tends to have effects on electronic equipment, such as audio and video recording devices. It is unknown if the effect is intentional or natural, though comments by the Observer of Noah's Formspring have suggested the latter. It could be due to the sigma radiation that he emits. Slender Man also has the ability to teleport at will, creating much distortion before he does so. It is noted that there is a consistent pattern of the video clipping at the bottom of the screen before he shows up.

Canon Aliases

• Mr. Slim
• Der Großmann (The Great/Tall Man)
• The Administrator
• Keeper

Appearances

• Introduction - In Memory of Milo Asher (Hidden in the picture of Milo Asher)
• Submission #1 (In the dark hallway door and in the reflection of the television)
• Submission #2 (After Milo starts running, he is seen walking out from the mangroves when Noah faces the camera back behind him)
• Submission #3 (In the static at the end)
• Submission #4 (At the top of the observation tower and at the top of the foliage before Milo starts running)
• Submission #5 (On the side of the road as Noah drives and outside when Milo films.)
• Submission #6 (In Noah's closet, in the doorways of the house during the odd footage, and outside of the hallway window at the end)
• Tape Analysis (Noah points him out in each of the Submission videos)
• Night Recording (To the far left of the shot of the pool during the lightning strike)
• The Unboxing (Only as a hidden drawing)
• Box Analysis (At the end of the tape after the person with glasses appears, Multiple times in the photos from the cell phone)
• Unknown Caller (Only tentacles)
• Nature Trail Visit (Appears at the end of a path and on top of the observation tower. He attacks Noah near the end of the video.)
• Secret Parent Interview (He can be seen from a distance, but then vanishes when Noah's mother starts talking about Karl.)
• INTERRUPTION (He is seen slowly walking towards the camera before lashing out with tendrils visible at the end of the video.)
• Northern Trip Footage (He is first seen on the plane Noah is taking to New Jersey terrifying everyone on the plane before suddenly vanishing and everyone, including Noah, forgetting about it. He is also seen briefly by Noah just before meeting the EverymanHYBRID crew.)
• November 11th (He is seen outside in the parking lot while Noah is in his car thinking he is going crazy and when the camera was on fritzing out showing the image of Slender Man and the words of the Boardwalk appearing.)
• Valentine's Day Livestream (Noah claims to see him and says that he's "wondering where he misplaced his face".)
• Mary Asher Phone Call (He appears in Noah's bedroom, and then later at the end of a hall after Noah looks out his front door.)
• The Order (He appears to the cult members and appears in front of the car to Chris, Alex, and Noah)
• The Live Stream Incident (As Noah/Firebrand goes back to November 11th, 2011, he finds himself entering as Past Noah freaks out. Slenderman rises from the floor and drags Past Noah away to his own realm)