What exactly was Roxas doing during Kingdom Hearts II?

[tevlen]

So, if Roxas was Sora's Nobody....

That means Sora was just a walking heart throughout CoM? He was a heartless with the shape of his original form?

Kind of, yes. You figure the keyblade itself is basically just given form by your heart, well Sora was a keyblade wielder, maybe his entire body was effectively given form by his heart, much like a heartless.

 

[Smoofy]

I beleive Sora's "fight" with Roxas, wasn't in his head, but occured within himself, the two fighting for dominance, trying to become the dominant being. Roxas, losing to Sora, realises Sora is supposed to be the one on the surface, succumbs to Sora, and let's him be who he is meant to be.