Mortal Kombat/Secrets

Mortal Kombat 1

 * Reptile (opponent)

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.