BIOGRAPHY.
Mortal Kombat Trilogy: "Born of Kitana's former world of Edenia, Rain was smuggled away from the realm as a small child shortly after Shao Kahn's take over. Thousands of years later he resurfaced. His allegiance belonging to Kahn, he chose to betray his homeland rather than suffer at the hands of Kahn's extermination Squads."
Armageddon Bio Kard: "Although the blood of Edenian royalty flows through his veins, Rain is a traitor to his realm. When faced with the prospect of death at the hands of Shao Kahn, he offered his allegiance to the emperor and betrayed his homeland."
Armageddon: "The Edenian race is descended from gods. That is the reason for our longevity, our nobility, and our strength. But I am more than a distant relation to the deities of old. I am a direct descendant of Argus himself. Only recently have I discovered my royal heritage. Quan Chi, an emissary of Shinnok, revealed to me that I am entitled to great power and control of Edenia. My father hid my identity for fear it would complicate his plans for succession. He favored his sons Daegon and Taven and groomed them to one day replace him as Protector of Edenia. But he foolishly tied this plan to a quest to save the realms from Armageddon, leaving open an opportunity for me to seize the power that is rightfully mine. Though it seems the Forces of Light seek to steal the prize from me, Quan Chi has made plans to assure my victory. He and his allies will help fend off the Forces of Light as I make my way to Blaze. But before this final battle, I must ensure that Taven and Daegon cannot challenge my ascension. I will finish them both before they complete their quest.
Mortal Kombat (2011): "A refugee orphaned by Shao Kahn's conquest, Rain grew up under the protection of the Edenian Resistance. An exceptional fighter, he soon rose through their ranks. As his reputation grew, so did his arrogance. When Rain demanded leadership of the Resistance forces, he was refused. Infuriated, he turned against his rebel comrades and betrayed them to their sworn enemy. In payment for this treason, Rain was promised his own army by the emperor Shao Kahn. Power would be his - no matter the cost... "

+
Nice juggle combos.
Good X-Ray.
High damage output compared to most Kombatants.
Has a medium kombo starter.
All of his special moves are useful.
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No low kombo starters.
Hard to learn for new players.
Kombos don't really differ from each other.

Button Translation:
1 - square (PS3)/X (360) T (Keyboard)
2 - Triangle (PS3)/Y (360) U (Keyboard)
3 - X (PS3)/A (360) G (Keyboard)
4 - Circle (PS3)/B(360) J (Keyboard)
Sorry, but I don't know the numpad button translation because I'm playing on the TUGJ variation.
Your main combo:
4 3 Down Back 2 (Lightning), wait, 4 3 Down Back 4 (Super Kick), 4 3 Back Forward 1 (Aqua Splash)
It does 37% damage, with JP (Jump Punch) 40%.
Alternative version:
Jump punch, 4 3 Down Back 2, wait, 4 3 Down Back 4, 4 3 Down Forward 3 (Water Bubble), (get enemy in front of you, but as high as it possible so you can continue the kombo) 4 3 Back+4.
It does 44% damage with JP. Sometimes you're not able to land Back+4, depends on water bubble position.
Alternative combo:
Jump Punch, 4 Down Back 4, 4 3 Down Forward 3, (get enemy in front of you, but as high as it possible so you can continue kombo) 4 3 Down Back 2, 4 3.
38% damage. If you're skilled enough you can land Aqua splash at the end.
Medium combo:
Back 2, 1+2, dash, 4 3 down back 2, dash, 4 3 back forward 1.
X-RAYS.
Same as the main combo, and alternative combo, but instead of Aqua Splash, use X-Ray, and then, 4 3. If skilled enough you can land Aqua Splash at the end.
ALL COMBOS ARE TESTED ON KEYBOARD AND ARE WORKING.
UPD: 23.09.2013
You can add Back+4 at the end instead of Aqua Splash. It's easier.

Tip #1
Use your Back 2 1+2 combo to start medium kombo.
Trick: If enemy is starting to block high after first hit, trick them and use 3 instead of 1+2, you will sweep instead of medium hit.
Tip #2
Punish Jump-Punching enemies by using your Geyser Kick.
Tip #3
Use your Aqua Splash against enemies like Reptile, if you hit them with it mid-dash, they'll stagger, allowing you to avoid taking damage.

Feel free to leave some tips in comments. Don't judge me if guide is not good, it's my first. Have a nice day.