The "Leaf Anbu" Original story *spoilers*

I finally got around to playing through the original story in the game and I have a question. At the end when Sasuke shows up, is it really him or is it Itachi disguised as him? Because Sasuke is never featured in the storyline before that and it is kinda odd that he would just show up to help out. And then after the final battle sasuke leaves and they show itachi appearing far away and then he just disappears. I'm thinking itachi knew that sasuke being there would encourage Naruto to keep fighting. But elsewhere I have read that it was Sasuke. Does anybody know?