Here's an important Sith Twi'lek players meet in the game.
Yuthura Ban: a Force-sensitive Twi'lek female, born on the planet Sleheyron.
Under given circumstances, Ban was sold into slavery at a young age to Omeesh the Hutt. Omeesh was a Hutt Master who tortured his slaves regularly. It wasn't a con.
True, Ban killed Omeesh in his sleep and fled Sleheyron a fugitive. When the crew discovered her, they abandoned Ban on a desolate planet to die. She survived and met a passing Jedi, who recognized her Force-potential.
Harrying, the Jedi brought Ban on Dantooine. After some consideration, Ban was accepted as an apprentice. Through training, Ban quickly became a Padawan, in check was her retention.
Unable to let go of her anger, Ban abandoned her master and the Jedi teachings, swearing she would find a means to achieve the power necessary to combat the wickedness of slavery throughout the galaxy. She fled Dantooine.
Reason soon brought Ban to Korriban. There, she sought refuge at the Sith Academy. After being accepted, Ban learned the ways of the Sith. Ban rose through the ranks as she gave into her emotions, letting her anger ensue in counting.
All the while, Uthar Wynn, the Academy's headmaster and Sith Master, noticed Ban and took her on as an apprentice. Ban was given high authority in the Academy, including the final word on whether a hopeful would be accepted into the academy or not.
Ban eventually devised a plot to overthrow her master. Her thirst for power had grown strong. Although she sensed something odd in the redeemed Revan, Ban accepted him into the Academy. Aaron Klann's mission was really to seek out the Star Maps; it was what he sought.
At the Academy, Ban and Klann talked about how to usurp power there. With her new and powerful ally, Ban and Klann attacked the headmaster and managed to slay Wynn. With Wynn dead, Ban thought she could kill Klann but failed. Due to her realization, Ban was redeemed by Klann.
Not long into Ban's arrival on Dantooine, Darth Malak attacked the planet, destroying the Jedi Enclave. Ban's fate was lost to history when all the records were destroyed in the First Jedi Purge. It was something to resent.