Slenderman's basic attributes vary greatly, however several basic traits are typical of his behavior. He is shown to behave in a very passive aggressive manner, stalking targets for years at a time, torturing his target mentally for various unknown reasons. He is rarely ever shown in a benevolent light and is typically shown as a malevolent force. This behavioral pattern has an uncertain reasoning behind it, and exactly why he behaves in such a way has yet to be fully ascertained.
The most important factor about Slenderman is the mystery. He lives unlike any human being despite having a similar appearance to one. It is uncertain if he is social, or even understands languages or behaviors, nor why it is humans appear to be his main target. His targets will typically behave in off ways as if possessed (Proxy), but whether their psychosis is causing them to speak for him or for themselves isn't entirely certain either.
Slenderman's appearance is variable as the years go on, but as of to date the most recent appearance is that of an abnormally tall human with long arms and no face whom is wearing a business suit. Again it is uncertain if his suit is real cloth or some form of skin molded into such an appearance. If it is the former, this would imply an intelligent being attempting to try to blend in. If it is the latter, it shows a being whom is extremely adaptive and evolves to match his environment and stick out less, or more.
Slenderman's appearance provokes a wide range of responses, the most common being fear. This is partially due to his extremely towering size over a full grown adult, which mimics the childhood fear of adult humans bigger than the child. His behavior also factors into it. Finally, the main fact is he is completely unable to be described in human terms. While a description in words can be given, human language lacks the ability to truly define the entity until such a time as when humans can interact with him (if at all possible). This problem stems from the fact that he operates in a plane of existence further from humans.