I'm not sure specifically as I haven't read it. But generally, it might be about the last passenger on a journey, perhaps on a ship, train or plane. It could involve mystery, adventure or some kind of personal struggle related to being the last one in that particular travel situation.
I'm not sure specifically as there could be many stories named 'the passenger short story'. It might be about a journey of a passenger, perhaps facing various challenges during travel, or having some unique encounters on the way.
Given that it's a 'passenger short story', the main character is most likely the passenger. But there could also be supporting characters. For example, if the story is set on a ship, there could be a captain or a crew member who interacts with the passenger in a significant way. Or if it's a story about a plane journey, there could be a flight attendant who has an important role in the story. Without more details about the specific 'the passenger short story', it's hard to be certain about all the main characters.