I've grown a liking to Raidriar (hence my user name) and I've noticed that his back story actually seems more interesting than Siris himself. So much that I wish we could play him to understand what happened.
A couple things that really spark interest is how he because immortal from being a mortal and his true (ultimate) intentions in the story. I think it's quite plausable the Raidriar is truly a good guy. After all, he tries to stop you from freeing the Worker who we know is evil. The IB seemed to be a way to protect humanity by keeping the Deathless in check. I think Siris just screwed over the humans lol. Raidriar did some evil by killing to power the sword but the point seems to be to finally kill the evil and corrupt Deathless (As he said in IB1 if you join him - which doesn't seem like a bad idea).
Actually the thing that makes me really want to have a game with a back story for Raidriar is when you speak to the librarian. When he talks about the great evil (not sure if that's his exact words) that used to be there but was slain by Raidriar on the Plains of Koroth I was thinking wow... that would be so awesome to play that out. It sounds like Raidriar is some kind of hero just by hearing that. They could definitely expand on that in the game or a new book (i hope!) and give him some more dimension.
After the end of IB2, it looks like Raidriar and Siris are going to have to get over their differences and start being bros