I remember when I was really young with no clue how to build my character... So here I was, with the crappiest character possible, fighting Sion, and it took forever to beat him. When I finally defeated him 1st round - with about 3 HP left - I went, "YES!" AND THEN.. he regened...
Anyways, I always thought of Sion as a harder Malak - unlike Malak, you couldn't stop Sion from regenerating. Not only that, but he hit harder. Storywise, to me, he was like Bane from TDKR.
Enduring pain, super strength, badass voice and power... For much of the game I was actually convinced that he was the main villain and there'd be this epic plot twist revealing every little secret about him. But felt his character was ruined by an old hag who rudely turned useful only AFTER leaving your party. >.> In the end, such an awesome character turned out to be merely a pawn.. which was kind of inconceivable. (If you watched the movie, you'll probably get my analogy.)