I was browsing the "Pugsy's Cheat" forums and came across this post by Tourniquet:

"As for Rain, he is simply a couple of prerigged moves for the intro as definitely not playable either CPU or human"

Someone should make a stickied thread dealing with older rumors about the older MK games.

http://cheat.retrogames.com/forums/viewtopic.php?t=1233&highlight;=umk3

EDIT : Also Noob Saibot IS playable on regular UMK3 machines, via these Cheats. He plays like a ninja obviously, and has no actual ending. If you beat the game with him you will get Human Smoke's ending. Also you can perform fatalities on him. I've tested more than 2/3 and they all work flawlessly. He is an incomplete character however having no special moves, not even Scorpion's spear or the teleport from the WaveNet machines.

Also when you select Rain on the CHEATS, the game simply crashes back to the desktop.