I have no idea, other than his keyblade is from the realm of darkness. Maybe Mickey walked the light path to darkness. That makes a little more sense, I guess. I am not saying my theory is right(it probably isn't), but it's the best thing I could think of.
I think that there are 4 roads. One is light, one dark, one is a dark road to light, and the last is a light road to darkness. Sora is light, Mickey is dark, and Riku is dark to light, but who is light to dark? I have heard that it is Roxas, but that makes no sense. Sora and Roxas wield the same keyblade, yet the other keyblade wielders(who supposedly walk different paths) have different ones. It could be Kairi, as she has a keyblade, but your guess is as good as mine. As far as BBS goes, it seems that it would be Master Xeahanort. That is my view on it.
