After you beat the game, the credits will roll in. Interestingly enough, if you close the DS and open it again, you can hear Vader's wheezing athsma sound.
Darth Vader for multi player
Beat the game.
Jedi Robes Costume
Beat the game with Light side choice.
Sith Stalker costume
Beat the game with Dark side choice.
Star Wars Knights of the Old Republic 2 Sith Lords Reference
When you go to the junk level for the first time, the boss is a short, stubby gray dude with a yellow double-saber. He's a reference to Bastila in the Sith Lords.
Various Codes
At the main menu, select the "Extras" option, then choose the "Unleashed Codes" selection. Then, enter one of the following codes to activate the corresponding cheat function:
Invincibility
Enter "QSSPVENXO" as a code.
Infinite Force
Enter "TVENCVMJZ" as a code.
Maximum Force powers
Enter "CPLOOLKBF" as a code.
Amplified lightsaber damage
Enter "LIGHTSABER" or "MOMIROXIW" as a code.
Kento's robe
Enter "WOOKIEE" or "EEDOPVENG" as a code to unlock Kento's robe.
Ceremonial Jedi robes
Enter "CURSEZRUX" as a code to unlock the Ceremonial Jedi robes.
Sith robes
Enter "HOLOCRON" or "ZWSFVENXA" as a code to unlock the Sith robes.
Sith Stalker armor
Enter "CPLZKMZTD" as a code to unlock the Sith Stalker armor.
Play as Darth Vader
Enter "HRMXRKVEN" as a code to unlock Darth Vader.
Play as General Rahm Kota
Enter "MANDALORE" as a code to unlock General Rahm Kota.