Mortal Kombat/Secrets

Reptile (opponent)
In-game clues to find Reptile and explanation:
 * "Alone is how to find me" (he can only be fought in Tournament Mode)
 * "Blocking will get you nowhere" (do not use the Block button at all)
 * "tip eht fo mottoB" (Bottom of the pit) (he's met in the Pit stage)
 * "Perfection is the key" (win a double flawless victory in the Pit)
 * "Fatality is the key" (use a fatality in the Pit)
 * "Look to la luna" (the moon) (if a shadow passes over the moon in the Pit he can be fought)

In-game clues about fighting Reptile:
 * "You must find me to beat me"
 * "You cannot match my speed"
 * "10,000,000 points if you destroy me"

Mortal Kombat 2

 * Jade (opponent)
 * Noob Saibot (opponent)
 * Smoke (opponent)

Mortal Kombat 3

 * Smoke (playable)
 * Noob Saibot (opponent)

Ultimate Mortal Kombat 3

 * Mileena (playable)
 * Ermac (playable)
 * Classic Sub-Zero (playable)
 * Human Smoke (playable)
 * Noob Saibot (opponent)
 * Rain (opponent)

Mortal Kombat Trilogy

 * Chameleon (not in N64 version) (playable)
 * Khameleon (N64 version only) (playable)

Console

 * Super Nintendo

Play as Human Smoke
Select Robot-Smoke at the character select screen. Before the match starts, hold +  +  +  +. Smoke will explode into his human form.

Play as Smoke
Push while holding the  button on the copyright screen. Then push the while holding the  button. Hold down the and  buttons until Smoke appears.