I'm big Star Wars fan, but I haven'y really read anything about those that don't appear in movies or series.
So maybe Darth Maul. His comeback story in Clone Wars and later Rebels is just so good. Such cool character with tragic story. Also goated voice acting. "Maul - Hatred" is such a good video.
A lot of people love nihilus but he literally has no personality, so I’d say Traya, her whole philosophy and reasons at the end of KOTOR 2 make her the most compelling bad guy of the E.U.