Well, that was vague
It wasn't "vague", it was something else. The school here is sort of the connection between life, and the afterlife. Graduation symbolizes moving on. The flower withering means that she wasn't truly dead, and since she's not dead. The place of the "dead" holds no place for her. It's implied all over that she's having difficulty breath, because that's how she went into coma. Monica realizes that she's dead when she turns into bones, and tells Fine that she can't follow her right now, but she's gonna keep the promise of "graduating" moving on together.
The end is rather dreadful, since both girls meet up, and you see Fine as a young girl rather than an old woman. Did she kill herself?
I take it as it is like a purgatory for young girls who died where they wait till they are emotionally/mentally ready to move on what is symbolised by them completing the flower things. When they complete it thats shows they are ready. Leaving the place the way they tried was them returning to the physical world and their bodies there. Since Monica was long dead her body was a decomposing corpse she was returning to and that is why that happen to her body.
After Fine had woken up she lived her life and died at a old age. Since the people in charge it all weren't assholes they let her become a young girl again and return to the girl who spent decades waiting for her friend to return instead of completing her flower ring and moving on. That is why they showed Monica with the long flower chain representing the long passage of time.as she waited. Now that they are reunited Monica can finally complete her flower ring and Fine can guide her to the other side as they go there together.
It would been better if they showed Fine as a old women first meeting Monica again then turning back into a young girl as they are reunited. To many authors are stuck on the idea they have make something vague with a abstract ending to make their story artistic and beautiful. Sometimes a nice solid complete ending can really be just as good if not better.
Edit: The flower ring on her finger was something she brought back with her from the other side showing her it hadn't just been a dream.
last edited at Dec 10, 2017 6:06AM