Itsuka, Nemuri ni Tsuku Hi

Hotaru Morino is a second year high school student who has had an unrequited crush on Ren Otaka since junior high school. One day, while on a school trip with Ren and her good friend Shiori Yamamoto, their bus gets into an accident. When Hotaru wakes up, a young man who is dressed fully in black appears in front of her. He is Kuro, a grim reaper. A shocked Hotaru shoves him away but he has shocking words for her: "I'm your guide. Hotaru Morino, listen carefully. You're dead."