This particular monster is a unique one sense theres only one of him (for now). A modern day boogyman, Jeff is a type of human monster similar to slashers from horror movies and serial killers in real life. In Crossover, Jeff along with Slender Man and all the other Creepypasta monsters, reek havoc on people where ever they go.
Descriptions by survivors mention Jeff wearing a gray, blood stained hoody, black shoes and pants, black, messy hair, and pale skin. Others have mention more shocking features regarding Jeff such as burned up eyelids, a slit mouth similar to that of the Slight-mouth Woman from Japan and tissue scaring on his body as if caused by a fire. Though people don't mention Jeff having any supernatural powers, those few who have encountered him and survived mention him having inhuman durability and some strange ability to influance people with weak psyches, almost like those other monsters who have power weak minds. People who have suscumbed to Jeff's influnce go mad and to some extent become "Knights" who act more like puppets to Jeff to do with as he please.
Some theories started revolving around Jeff regarding weither he himself is a mad man who, as the story goes, went mad after a freak accident during a party and killed his entire family or may be something more inhuman in origin. Few people have suggested that Jeff may infact be a type of being who is the lving embodiment of fear involving madness, serial killers, and intruders. A being called a Fear, other beings such as Slender Man are considered to be Fears as other theorized but theres not much data regarding the fact.
Note: Along with the Creepypasta stuff, I'll also be adding other Creepypasta-related spinoffs such as the Fear Mythos and the SCP Foundation Mythos too. Frankly I've already got some story ideas involving Jeff, Slender Man, Smile Dog, the Rake, the Fears, and the SCP Foundation in later projects.
Jeff (c) to whoever made him