Just picked this up, so I'm late to the speculation train, but it's interesting that in the sea chapter, Chiaki says that it feels like she's been on the car before, and she drinks a certain drink. She also tells us that her father says policemen are not to be trusted. The same drink that she drinks is the one that is put with some flowers on the side of a road, by a person who calls policemen pigs. Chiaki definitely doesn't feel as though she's alive, perhaps a spirit or entity that can only be seen by those who want to court death? That might also explain why she says she is his lover. Maybe she is the grim reaper in this story, or a different entity, but this story is pretty interesting.