To be completely honest, after squinting at that picture for ages, I realized that it's impossible to even begin to speculate just who is sitting with Naruto. Because the background is already there in detail, and the person is just a vague outline, it is literally impossible to tell who it is. I personally couldn't tell if it was hair shading, or details on the back of the shirt (like Sakura has), because as kira pointed out, the second figure's head is pretty much all over the dark portion of the background. Also, the second person's hair is definitely not shaded in on the bigger part of the picture (where you can see the heads on the hokage mountain).
It would make the most sense for Naruto to be talking with either his parents, or Sakura there. Sakura, because they need to be alone to discuss getting out of the genjutsu, and because they want to get away from crazy genjutsu versions of the people they know, and his parents because they are also main characters and he never really got the chance to simply talk with them (though if this were the case, why not talk with them at their house?).
tldr: it's impossible to say for certain who's sitting beside Naruto at the Hokage mountain because (a) its tiny, and (b) its a vague sketched outline on top of a detailed background.
Edited by Paptala, 01 July 2012 - 04:57 AM.