Sion is my favourite baddy in the game I would have liked to see more of him and see him again in the future. He was originally compared with Malak but he is nothing like Malak IMHO. He always intrigues me and I love the tension between him and Kreia.
Nihilus was an interesting idea but under developed and Krei has a lot of knowledge and teachings to impart but I have to admit as to being left confused by her at the end and reverting to ignoring her on most play throughs except to get what I need out of her.
Although I'm playing again right now and might use her a bit off the ship more then I usually do.