Ai Yazawa tragically ended Nana/Ren's romance on purpose
This author have issues in allow her heroines to get the guys (remember Paradise Kiss?). She keeps ruining the main romance in her stories. This time... poor Ren.
- Yazawa also doesn't like her girls to get the girl. I still contend that the main romance here is between the two Nanas, but Yazawa keeps putting guys in between those two. It irks me how she first puts so many romantic overtones between them, only to drop the issue like a hot potato later.
Nana O is dead.
I think she is dead, six years later, that's why Hachi narrates the story all the time talking to her.
Nana O is in a coma.
Nana K does narrate in such a way that one could come to believe that she is dead; however, in volume 8 Nana K says: "Even now I keep calling your name/despite my pain/I'll keep calling you until you answer me.". Based on that, it seems to me that she's simply in a coma.