After the successful fiasco that was Project Lumen, I've had time to recover from what Lucia insists on calling, and I quote, my "entirely fucked up, out there BSOD." Now, it is time to do a collective profile of what is commonly known as the Slender Man, based on my own observations and findings.
First off, back when I first met him (taking into account my collective year and a half in accelerated time) he introduced himself as Ola'asu, a name bearing a strong resemblance to H.P. Lovecraft's monsters, although this is only a loose comparison.
Secondly, in meeting him, I noticed that he lacks a mature sense of humor, along with many other 'mature' traits, acting something like an eight year old. This is especially present in his always revealing his presence to his target instead of killing them before they can report anything. While these reports may be the exception to the rule, there is still the noticeable number of revelations prior to any hostile action that shows in all these accounts. These indicate an interesting desire to be noticed and seen, despite the fact that this limits his mobility (more on that later).
Now, I'm not the first to propose that Ola'asu might have an infantile or childish mind, but that is not without reason. Ola'asu is known for his removal and haphazard replacement of victim's internal organs, much akin to a child pulling legs and wings off of insects. However, he also demonstrates an adult or, sometimes, above-human capacity to predict behavior and other psychological results that are normally too chaotic to reliably predict. Take note of how he broke down Damien here: http://dreamsindarkness91.blogspot.com/2010/08/he-was-right-here.html by assaulting his normally rational and logical outlook (even incorporating his insanity, his 'Damien' aspect was an anchor, so to speak) with irregular information, sometimes falling directly into the common mold of his abilities, and sometimes directly contradicting it, such as teleporting in plain sight. This was likely a deliberate trick to break apart what sanity Damien had, possibly utilizing blackouts to achieve the effect.
Third, I will list off what I know / can confirm of his physical attributes. The most obvious are, of course, his appearance as a disproportionally skinny, bald, and tall man with no visible facial features and extremely pale skin, dressed in a business suit, black pants and jacket with white shirt, matching his head. He is able to alter his body proportions at will from nearly-human to somewhere around telephone pole-height, with only his head and the white shirt portions of his body maintaining their size. He mainly stretches lengthwise (his body gets longer, not broader). He can also, at will, sprout and undefined (yet symmetrical) amount of tendrils from his torso's side and back, as well as converting his arms into such tendrils. An interesting thing to note is that in most cases, Ola'asu's business jacket also extends to an wraps around his hands, glove-like, and also accommodates the tendrils from both his torso and arms, yet I know for a fact that it is an otherwise ordinary suit jacket. Ola'asu has never, to my knowledge, changed his legs into tendrils, though, for reasons I can only speculate at.
Along with his bodily manipulation, he is capable of supernatural acts. Ola'asu is most noted for his ability to move at near-infinite speeds when nothing is making visual contact with him, even (as I've recently confirmed) in indirect ways, such as his shadow. The threshold for this, however, seems to be limited. For example, blinking has never presented an opportunity for Ola'asu to appear, but turning around from a window you had just seen him through can. He is also shown as capable of causing humans to go unconscious, seemingly at will, however, he rarely utilizes this, possibly due to some limitation I cannot arrive to at the moment. While unconscious, the person can be forced to act under Ola'asu's orders, or simply left incapacitated, usually with no recollection of events afterward. It is entirely possible that most proxies are simply blacked-out humans under his influence.
Ola'asu has also been known to cause a unique illness in people who have been subjected to his presence for an extended period of time (not necessarily all at once). The symptoms usually involve coughing of increasing severity over time, and of increasing debilitation as Ola'asu gets closer (for example, an affected person will get a gradually worse cough from Ola'asu's presence from 20 ft. away, but anyone with any severity of the illness will instantly have a racking coughing fit, forcing them to their knees, any time Ola'asu comes close to them, though the initial coughing symptoms still need to be present.), as well as a constant chilly sensation.
Ola'asu also seems to be able to obfuscate himself from view. That is, he does not become invisible, or even blend in. He simply fades from notice, but in such a way that one must have seen him by 'accident' to see him at all. For example, someone who had never heard of the Slender Man stories might think they saw a tall, bald, pale man in the grocery store, but that does not mean they will see Ola'asu himself as he is. However, someone who has watched all the Entries of Marble Hornets might see the same figure and see it as Ola'asu, and unfortunately be entirely correct.
Then there is his interference with electronics. Any electronic device that takes a live video feed or creates sound is prone to distorting in Ola'asu's presence, possibly due to the same effect that causes the sickness in humans. Note that I specifically said 'live video feed', as still shots come out with no distortion beyond the shot's own blurriness, while videos viewed after the fact or even video viewed as it is being recorded all distort drastically. This leads to my thought that Ola'asu emits a low-frequency radiation, showing in electronic A/V feed as distortion and in humans as mild radiation sickness or cancer. However, the two could be unrelated, and the video distortion is a deliberate attempt to destroy one of the eyes holding him in place, with the audio distortion as a side-effect (or, knowing evolution, it could be both, the radiation serving to disrupt digital eyes and debilitate biological foes).
Finally, the Operator Symbol. My initial thought upon seeing it was "the Celtic cross" which is formed as a plus inside a circle, instead of an 'X'. This train of thought did, however, lead to an interesting discovery. The Operator Symbol has been used in astronomy since astronomy was first made into a science. The symbol for the Earth is the aforementioned Celtic cross, while the same sybol turned 45 degrees is the symbol for Earth, with its axis turned 45 degrees: http://en.wikipedia.org/wiki/Astrological_symbols#Miscellaneous_symbols
This symbol seems to have an almost runic power to it, where it's function is to affect Ola'asu's distance to it, and its actual effect is determined by the intent of the drawer, but not in as straightforward a way as normal runes. For example, it has been known to both summon and repel him, while being intended to repel. However, without knowing for sure what was intended by the writers, there is no way to know how to produce either effect consistently. It has also been known to be carved by Ola'asu himself, seemingly to neither effect, simply serving as his calling card of sorts. There are reliable ways of summoning him, however, such as doing something specifically to irk his immature aspect, such as playing the $20 song as loud as possible in direct defiance of him (See also: Project Lumen), however the point is much more in finding reliable ways to repel him. Very few things are found to work consistently, assuming they work at all, such as Damien's bone. It seemed to work to repel Ola'asu, but he was just as prone to pick it up as flee from it, as detailed in the link above.
The next topic is that of what will simply be called Fear, the creature of sorts residing in Lucia's mind. All I know for sure is that its ability with runes stretches beyond everything I know about them. To my knowledge, it shouldn't be possible for an outside person to modify a runeset and it still be able to be used as-redesigned by the original creator, much less in an instant, however, while I attempted to examine the beat itself, it modified the runes I was using to look into Lucia's mind into farces of runes that spewed sizzling bacon everywhere. This tells me three things about Fear: One, it exists on the exact same level as runes, that is, a physical manifestation of an abstract concept, only existing due to being created, and only being created to invoke the concept it represents. Two, it has a sense of humor comparable to a four year old. Three, it is capable of sending me into rages more capable than all the militarized runes I keep in storage combined.
That said, I have spoken for long enough, and I am not as used to these long-winded posts as Lucia is, so I will end this in a somewhat cheerful way with a video of the opening to my favorite series in the years before the crash. Hopefully it isn't too out of place here.