I would say the main peculiarity of the diary in The Diary of a Space Traveler is the fact that it came back to earth without its owner. It was written by Professor Shonku who built a rocket to go to Mars and he did, but his diary was found on earth even though he wasn't.
The referring to one self with pronouns such as "I" or saying phrases such as "he told me" along with narration of personal thoughts (but that is contextual)
