In Mortal Kombat (aka Mortal Kombat 9, 2011 version) for the PS3 and Xbox 360, there are four hidden kombatants (as they call them) similar to the old games that you can encounter during arcade mode by meeting certain requirements on the match. Each one of these will unlock a corresponding hidden achievement/trophy.
Fighting Hidden Kombatant Classic Jade for the You Found Me Achievement/Trophy
With a minimum of two round matches, don’t lose a single round up until the Shang Tsung fight. On the Shang Tsung fight, get a double flawless victory against him, and then perform a Fatality on him, and Classic Jae will come out to fight you on the Goro stage.
Fighting Hidden Kombatant Classic Noob Saibot for the Brotherhood of Shadow Achievement/Trophy
When you see Noob Saibot in the background of The Temple stage, win that battle without using any blocks at all and you’ll fight him.
Fighting Hidden Kombatant Classic Reptile for the Pit Master Achievement/Trophy
Look for a shadowy figure to fly across the moon on The Pit 2 Night stage, then get a double flawless victory and perform a stage fatality, and he’ll come out afterwards.
Fighting Hidden Kombatant Classic Smoke for the Hide and Seek Achievement/Trophy
Look for Smoke to peak out from behind one of the trees on The Living Forest stage, and immediately press Down plus the Back button on Xbox 360, or Select on PS3. Do this repeatedly and quickly, and Smoke will come out to fight you.
