I found all of them intriguing (with the exception of Qordis, perhaps) but I like Kas'im a lot, especially in regard to his relationship with Bane. I enjoyed reading about him training and mentoring Bane and I found it rather a pity how their relationship ended. Though since they're both Sith it was probably unavoidable that they'd end up trying to kill each other.
I liked Githany as well, though. I found it honestly hard to tell when she was acting and when she was (more or less) sincere. I also enjoyed her and Bane's relationship