I've been planning a game for Halloween with Slendie as a "villian." I haven't gotten around to statting him yet, but I conceived of him as a scion of a Wyldfae associated with the Erlking. Both of those characters have links to Germanic folklore, both of them are seen as beings who steal, terrify, kill, or are otherwise associated with children, and one of the first things that you read about scions in the novels (in Blood Rites, if I recall, when Kincaid is first introduced), is that they tend to be insane, first and foremost. That seems to be a pretty good definition for Slenderman.
Of course, the simplest way to make him (and probably best way to kill your players) is to make him an Outsider, as that's clearly what he is.