Kitana is essentially the "good" version of Mileena (or, more accurately, Mileena is the "evil" version of Kitana, if you're super particular about your Mortal Kombat nuances) - a mostly-undressed lady ninja who covers her mouth. Although, lucky for Liu Kang, she's not hiding a demon mouth under there. It's sorta miraculous she's as good as she is when her primary weapons are just fans (METAL FANS, I KNOW). If nothing else, she's definitely ready to cool herself off on a warm summer day.
Smoke made his debut as a secret fight in Mortal Kombat II, essentially a grayed-out Scorpion (noticing a trend here?) with puffs of smoke coming out of his body (the assumption being that if he doesn't kill you in the ring, he'll get you with second hand smoke eventually, I guess). He then spent a little time as a cyborg (he was the gray one, with smoke coming out), before returning to his human form in 2011's Mortal Kombat - except now he had glorious, glorious hair. He's basically this series' Don Draper, since he's all about smoking and having great hair.
The first intentional hidden character in the Mortal Kombat series, Reptile (you guessed it!) a palette-swapped version of Scorpion (with a few Sub-Zero powers thrown in), only accessible through a double flawless victory on a certain stage ending in a fatality and ONLY if a shadow passed in the background. In other words, it was the stuff of grade school rumor legend. But after that, he became a regular character in the MK roster and got his own set of abilities - spitting acid, turning invisible, and eventually turning into an actual lizard-dude. Not bad for the laziest-named character in the entire series.
Not only has Shao Kahn consistently been one of the hardest boss battles in a Mortal Kombat game - he's been one of the hardest bosses in any game period. And for someone who is supposedly an incredibly powerful conqueror (EXCUSE ME - "Konqueror"), he sure does think dressing like an S&M version of the Conan is a good battle strategy. Probably the coolest ("koolest") thing about him is that his voice is actually that of the commentator/announcer, which can at least distract you from the fact he looks like The Rock got drunk in a Halloween costume shop.