i'm having a scary premonition.
they go on the mountain trip, but danoh's heart stops during the trip, and that's why haru looked so angry at that poster. he was also holding vol. 4, which is the missing volume where danoh is supposed to die. danoh is supposed to die on the mountain trip, but like
@ch0si says, haru dies in her place to give her his heart.
i really hope my guess isn't right, i need this couple to thrive so i can sleep well tonight but ha ha looks like i'm crying myself to sleep again