On Dantooine during your Jedi training you will be sent to investigate someDark power emanations in the wilderness. When you find Juhani, you can turn her to the Light Side and have her join your party, or you can threaten to kill her. Do so and you will set off an interesting chain of events that not only lose you Juhani as a party member (since she's dead) but sends another Jedi member towards the Dark Side.