The bonus stage might actually give us a glimpse in to who friend is (it seems to be a virtual reality thing of the city they used to live in ... So it would make sense that you would be seeing it through the eyes of friend, after all their will be certain things that "friend" didn't know about, like maybe a conversation w/ Donkey and Otcho or perhaps that Kenji's bike was broken, things like that, and then perhaps a flash back later on will tell us who friend is)