Hidden players?

I'm very positive there will be, probably unlock as you progress through the game.

SPOILER:

Quan Chi is a character unlocked through this way

Obviously yeah & some dlc so look forward to it.

Yes, the unlockable characters are quan chi and cyber sub-zero, but there is also 4 fighters you can fight that arnt playable, such as noob's shadow. to fight the unplayable fighters, you have to to fight on a specific stage, for example, if you fight on the evil monestary, and noobs shadow is in the background, you must win the fight without blocking to face him. im not sure if you get rewarded for beating them, except for the achievment of course.

You can fight the MK1 reptile by getting a double flawless in the pit and doing a fatality (I did the pit fatality). He gives you 1,000 koins for each round you lose, and 5,000 koins for each round you win.