Todd McFarlane's iconic Spawn is one guest character that both Mortal Kombat hardcore and casual fans can agree upon.
Very few superheroes become iconic outside of DC and Marvel. In fact, you could list them on one hand. Well, Spawn is certainly the most famous of all non-DC and Marvel heroes, and his popularity reigns from his ultra-violent nature; think Batman meets Deadpool.
Spawn is a tragic character, as a man who was murdered, then made a deal with a minion of hell to be able to go back and see his wife. The minion tricked him however, sending him back to Earth with no memories, as Spawn, to do his bidding. Shortly thereafter he remembers his past and seeks out his wife, only to learn she married his best friend.
With all that in mind, it may be more understandable why Spawn is so incredibly vicious. As an undead assassin, Spawn is pretty damn hard to kill, and has excellent fighting skills to boot. He's also fond of a gun, chains, can breathe fire, shape-shift, teleport, and even summon warriors from hell to help. It ain't hard to see how all that fits neatly onto the Mortal Kombat roster.
The exciting news is, Spawn creator Todd McFarlane stated last year that he thinks the character is on his way to Mortal Kombat. So perhaps this one is a matter of time.