This page gives a detailed list of all of the traits and abilities shown by the Slender Man.
Slender Walking
One of Slender man's most infamous of abilities, Slender Walking, is an ability of the Slender Man that allows it to appear and disappear at will, granting it omnipresent-like movement, allowing it to teleport from one point to another virtually anywhere instantly and often without any physical indication of the teleportation.
The ability is often explained in a variety of forms, and depending on the depiction in that medium, it may be substantially different (i.e., a paranormal explanation rather than a scientific one, or a demonic explanation rather than an alien one, etc.). The theory of Slender Man as a Fourth Dimensional Being is one of a few explanations to try and explain this using the Quantum Theory, but to date, no factual statement has been made that fully determines this explanation.
Camouflage
The ability of the Slender Man to disguise itself is one of its more important traits. The most well-known camouflage is that of the contemporary suit-wearing Slender Man. The most supported theory is that the Slender Man takes the appearance of a member of higher class, rank, or notable social position (a knight, a warrior, or a suited man), in order to gain trust or acceptance from a person, or at the very least to blend in at a distance. This was based on evidence drawn from a 16th century woodcut entitled "Der Ritter," which depicted a knight with elongated limbs believed to be The Slender Man. A variation of this theory is that the Slender Man's appearance varies from person to person slightly, appearing how the individual's mind comprehends it.
It also should be noted that in the early mythos, Slender Man appeared to have a face, but it did not appear on film. This seems to suggest that he can make himself appear to have a face, but that this illusion, while visible to the human eye, does not transfer to film. This is a possible ability he could use to blend in, as a faceless person would stand out a bit and onlookers would start panicking, allowing his target to escape in the ensuing chaos.
In some pictures of the Slender Man in a forest, he can easily blend in with the trees in the background, sometimes using his tentacles like branches.
Mind Control
The loose defnition of "Mind Control" is utilized and assured by many series across the mythos and perpetuated in blogs and video series. Exactly what this mind control entails is extremely loose and very poorly defined, although it is always an inevitable topic when discussing Proxies, a term coined by DarkHarvest00.
As most series state, proxies work on behalf of the Slender Man, who controls their thoughts and feelings, and through that, their actions. Proxies may still have free will, or partially free will, and may or may not know that they are being controlled. They are used to interact with other victims, set traps, or give information to the victims on behalf of the Slender Man. It is debated as to the full independence of proxies, and some suggest that when under its control, proxies act as a collective hive-mind. Examples include the Collective of TribeTwelve. It is assumed in some iterations of the Slender Man that he is capable of puppeteering and controlling the proxies directly; however, such evidence has never actually been shown in a Vlog or Blog, only assumed.
Kate and Charlie Matheson Jr. are examples of in-game proxies, from Slender: The Arrival.
It is important to note that Proxies are not used in the series MarbleHornets, and the major antagonist is The Operator, thus this section has no correlation with that series.
In Slender Rising 2, the player can find a shotgun to defend themselves from the Slender Man in an otherwise lethal situation. If the player has the shotgun equipped and Slender Man isn't around, the player may suddenly find themselves aiming the gun at their head at random, with static appearing in a similar manner to the moments where the player sees the Slender Man. If the player doesn't break free, the player will shoot themselves.
Impersonation
Rarely seen in the contemporary mythos, early mythos accounts show the Slender Man's ability to impersonate or mimic the voice and tone of a person, in order to get to a victim. The most common use is voice mimicking, for the purpose of luring a child or victim out of a safe area. He is also able to produce the sound of a child's giggle, perhaps to draw another child in.
Tentacles/Limbs
Another of its famous abilities, The Slender Man's human-like appearance often disintegrates upon the appearance of its tentacles or limbs (also to be referred to as "tendrils"). While appearing far less in contemporary ARE, the main exception being TribeTwelve, the tentacles played prominent roles in the Original Mythos. The Slender Man's tentacles number into the tens regularly, and it is unknown how many it can spawn, usually from its back or shoulder, though it most commonly spawns about six. The tentacles themselves are used primarily for reaching, grabbing, or overtaking obstacles, and have been shown to be rather powerful. Some Original Mythos stories and images make use of the Slender Man's tentacles for balance, walking/supporting, or climbing, not dissimilar to a spider's legs. This is one of the only other ways the Slender Man has ever been shown to move besides Slenderwalking.
During Chapter 5 of Slender: The Arrival this is one of the ways he will capture Lauren on her way to the Radio Tower, by actually spreading out his tendrils throughout the forest in attempt to block any means of escape.
Selective Visibility
For more information see: Distortion
One of the enduring traits of the Slender Man is selective visibility. The Slender Man potentially has the ability to choose who sees it and when, in many cases. For instance, it can allow only one person to see it, or only the prey, but not anyone else who would be looking at the same place. Often, young children can see it when adults can not, and it is commonly agreed that even when humans can not detect it, it can be seen by cameras, both digital and pre-digital, although there is often distortion when captured digitally.
The only problem with selective visibility is that it makes it nearly impossible to discern who has actually seen Slender Man, and who might be making up tales for attention. While this ability is generally regarded as factual, it has not yet been proven true and the chances of selective visibility ever being proven true are very slim.
Some suggest that animals are receptive to the Slender Man, and that the Slender Man's selective visibility is not effective towards them, and it therefore kills animals that pose a threat to its hunt. This could be an explanation for the appearance of bags filled with animal corpses in early mythos.
Fire
Early mythos and some ARGs have strong connections between Slender Man and fire. In the Original Mythos, almost all Slender Man visitations are accompanied by fire, either after or during. He may be able to use pyrokinesis, which is the ability to move or control fire with one's mind. Often, the fire kills witnesses, or destroys evidence of its existence. Photography of the fires often show the Slender Man in or around the fire.
In some cases, the fire is blue and on rare occasions green. It is not known why this change in color occurs or what motive any of the fire has.
Mutilation
A common early trait of the Slender Man was that of body mutilation. A common procedure would be the discovery of a missing person's body impaled on a tree limb, high above the forest floor. Medical evidence would suggest that the person was often alive during the impalement; however, bodily sensation would likely have ceased quickly from blood loss depending on the impalement location. This was often accompanied by the removal of organs, which were then placed into plastic-like bags or sacks of unknown origin, and then replaced back into their original positions in the body. This was usually seemingly done while the victim was alive; however, removal and physical contact with organs would most likely have sent the victim into shock or killed them in seconds, rendering a fast death.
Also, alongside the desecration of victim's bodies was the mutilation or death of animals, usually cats or small animals, but occasionally dogs, for reasons unknown. Often, mutilated animals would be left in plastic sacks or bags, similar to the organs of victims, in the forest or around victim's homes or where they would find them.
Exactly how he was able to cut open a person or animal's body is unknown, but it may have likely been the use of his bare hands; his fingers are often long and bony with a sharp looking tip, it is possible that the tips of his fingers were sharp enough to make incisions into soft flesh.
Sickness
Main page: The Sickness
In several series and ARE, characters in contact with or in some form of relationship with the Slender Man may come under sickness. The sickness causes coughing, bleeding from the mouth, nose, or eyes, vomiting, disorientation, potential black outs, and potentially fevers which first appeared in TribeTwelve and can now be seen in MarbleHornets and MLAndersen0. In EverymanHYBRID, the sickness is attributed to Sigma Radiation, which is assumed is released by the Slender Man. This may make it a form of radiation poisoning.
It is important to note that outside of ARE's and ARG's, very little data exists on how the sickness works.
Weather/day Control
In many of the Slender Man games, the Slender Man often stalks the players in weather or day conditions that cause fear, such as the night, fog, rain, or storm. Also, in Slender, fog grows thicker when the player collects more pages, thus making it more possible that the Slender Man has some degree of control over the weather.
In the beginning of Slender: The Arrival, as Lauren gets closer to Kate's house, the time of day seems to rapidly go darker, eventually turning to night as the player arrives at the house, this may indicate that Slender Man not only is able to control the weather, but is also able to accelerate the day and night cycle to a selected time of day.