What is the writer or narrator's perspective: an older reflective self, a central player, a witness?
Of the three choices listed in the question, I would choose to refer to Jon Krakauer, the author and narrator, as an older, reflective self. In Into the Wild, Krakauer refers to his own experiences and perceptions at length, rather than just revealing the facts of the life and death of Christopher J. McCandless; this knowledge should lead the reader to understand that Krakauer does not consider himself, nor represent himself, as merely a witness. In addition, Krakauer had not input into the events of McCandless's life. In fact, Krakauer never even met McCandless, so he could not be considered a central player, despite including some of his personal experiences and opinions in the content of the book.
Krakauer related to McCandless and shared some common or similar experiencesa with the younger man. He appreciated the efforts McCandless made to achieve his goal and seemed to do his best to defend the actions of "Alexander Supertramp." However, he does not totally excuse the poor behaviors and multiple oversights of McCandless as a family member or naturalist. This leads the reader to believe that Krakuaer does indeed consider himself an older, reflective observor of the events of McCandless's brief existence.