How do you get the last charachter on Star Wars the force unleashed

the last character is starkiller (the apprentice) in his sith stalker armour. In order to obtain this character you must play the last mission on the death star and instead of attacking the emporor when you have the chance attack the already beaten Darth Vader. this will start the alternative dark side ending where starkiller becomes the emporors apprentice and gets life sustaining armour like Darth Vader.