He's got white blood in the series (or part of it) so I'm thinking he's gone cyborg on us. But as for being a main character I hear he is going to be controlled mainy as an AI to aid Snake. I don't know if you've seen the
E3Trailer or not but he is definitely kicking some ass. This is supposed to be the last of the SOLID games so Raiden is most likely going to be in the next few without Kojima or Snake being involved.
I need a new hobby.