They've just teamed up to get to the final goal. They're using each other. When they finally get close to that goal, all sides will be battered down to their last members, and that's when they'll turn on each other.
I suspect that Aizen will be the final enemy. That or the Soul King himself will turn out to be a dick as well. Either way, I see Juha as being very strong, and a likely semi-final boss, but definitely not the final itself.