Jeff the Killer is not a real person. The character first appeared on October 3, 2008 in a video uploaded to YouTube by Sesseur. Jeff is frequently featured in "creepypasta" -- scary stories that are shared via the Internet.
Jeff the Killer is depicted with unnaturally pale skin and long, dark hair. His most prominent feature is his wide, heavily mutilated smile. According to the original Jeff the Killer video, Jeff appears deformed because he spilled a container of bleach on himself while cleaning the bathroom. Jeff is associated with the phrase, "Go to sleep" and is often shown wielding a bloody knife.