He behaves in what might be seen as a passive-aggressive manner, often stalking targets for years at a time before ever attacking. It’s understood that he tends to torture his targets mentally, over long periods of time, using fear and paranoia to drive them to insanity. He brainwashes children, using them to further his own (unspecified) ends as Proxies.
However, his basic appearance and traits are relatively consistent across different accounts. His appearance has changed over the years, but the most common image – and the most well known in media– is that of an abnormally tall human with long arms and a totally blank face. He is usually portrayed as wearing a business suit, which may be either real cloth, or some form of skin molded to take on the appearance of cloth. If the former, this would imply an intelligent being attempting to try to blend in. If it is the latter, it suggests an extremely adaptive being that evolves to match its environment.