Really, the only one of these criteria which anyone can agree on is His appearance, and even then there's variation. The only consistent details are as follows:
Bald, with a blank face
In a business suit (contemporary) or other authoritarian dress (historical).
A lot of the time, He is described as having disproportionately long limbs. Some accounts even talk about Him having tentacles, but that seems...a little preposterous to me.
Other than that, some describe Him having the indents of a face, others having the front of His head be perfectly smooth. Height differs anywhere between six and ten feet. Thinness differs from thin human to impossibly emaciated.
As for powers, these differ wildly. Firestarting, out-of-sight teleportation, time/space manipulation, hypnotism and mind control, amnesia induction, and more. Any one account could attribute The Slender Man with any, all, or none of these powers. More universal ones are an effect on electrical objects, particularly video cameras. Video cameras distort in His presence - some subtly, some wildly. Note that still cameras do not - perhaps either His movement, or more than an instant's perception, causes this effect. He also seems to possess the ability to zone in on people who know about Him - the more, the better.
As for His nature, it is this which changes the most. He has been interpreted as a wild, instinctual creature, a coldly sapient puppetmaster, a frenzied killer, even a minor God. He has been interpreted as working alone or as a puppet of a greater force. He has been both capable and incapable of communicating with humans. He has been ascribed hominid motivations, or simply stated to have motivations beyond the comprehension of humans. He clings to his stealthy, stalking demeanour with wildly varying degrees of tenacity.
Once again, all these show us is that, for all that has been written, we know so little about the Slender Man that it's laughable. Daddy, as I know Him, could be assigned some of these traits, fits within certain parameters, and ignores others. Much has been written - an enemy we know things about is an enemy we can defeat, so said Robert - but almost all of these seem to have been vain attempts to rationalise a phenomena the writer had no real comprehension of. As with many topics - natural selection springs to mind - everyone thinks they understand the Slender Man, but their observations are generally lacklustre at best.
Let's hope mine are not in that category.
About halfway through writing this, the words felt like they stopped meaning anything. Meaningless symbols in an arbitrary order. This whole time, I've been looking for a way out. For Kari. Nothing's been found, but she's dead. There's no reason she'd be alive when the others died. And the part of my brain telling me this hates the modicum of hope which still remains. I wish I could say "Nothing's worse than not knowing," but that would be lying.
Since I met her, I've never gone this long without seeing her, talking to her. Seeing her smile. Telling her I love her.
We hit it off basically the day we met. We've never not been in love.
And now she's gone, and what was her is now a hunk of meat, rotting by a roadside somewhere. The eyes I could stare into for hours, the body I felt press against me when we were alone. The lips. The mind. The soul. The voice. It's all gone. And I'll never make peace with that fact.
And of course, He's pretty hard on my tail right now. Fucking with my mind. Perhaps rather callously, I'm basing the time I have left who knew about Him before me. That list of human shields is becoming shorter and shorter. Assuming I last that long. I'm feeling myself slipping. The lack of sleep. I'm imagining things again. That whole Steven Taynor thing.
If Kari and I had never met, this would never have happened. If Kari hadn't been assigned Joey's class, this would never have happened. If Joey hadn't found out about Daddy, this would never have happened. If the two people who would go on to become my parents had, just by chance, never met, or Kari's, or Joey's, this would never have happened. And yet, through a series of random events, this whole thing came together, leaving me typing on my phone about being stalked by a supernatural hunter-killer after days upon days of no sleep.
Victims of chaos.
I've found myself thinking of that damn dog.