If you wanna know what Slender Man and Smile.jpg are, research them.
Yep, it's Slender Man, the infamous monster/demon/alien/fallen angel/djinn/fairy/whatever... he's an eldritch humanoid... most known on the internet than anywhere else. But since this is an internet memetic character, if he were to be a guest for Mortal Kombat, he might as well strictly be a DLC and so whould his stage (if it's possible for there to be DLC stages). He'd also end up as a joke character like Mokap and Meat, but he definitely won't have so much hatred like Mokap and Meat.
*Being a tall dude, he's as big as Goro and Kintaro, only he's not muscular.
*His stage is based on the smile.jpg image which has a reddish color and a bizzare doglike monster with bug-eyes and human teeth.
*Some of the stuff about him is based on interpretations on him.
*The 2nd costume's description is just an idea of mine.
Slender Man © TEH INTERNETZ!
Mortal Kombat © NetherRealm Studios and Warner Bros