This has to be one of the dumbest manga's I've read with such a high rating.
The MC asks if it's strange to love a ghost. UHHH YEAH? What do you think you're going to be able to experience with something that can't experience anything? She will perpetually be 15. How's he going to feel when he's 30? Does he realize that she's 65+ years old mentally? How's that going to work? They can never go out, they can never have children, they can never enjoy anything together. He can't even be certain that's the real her or just that fragment of her split memories. When she merged with her grudge, she appeared to have had a personality shift. So how can he be so sure that's the woman he loves and loves him? They can't even consecrate the relationship since she's ephemeral.
The REALLY ANNOYING part of this manga is the stupid harem that the MC builds and the forced romantic relationship between the dead for 50 years girl and 15 year old annoying reader stand in. The only good character was the great grand daughter. She was actually decently written and had a reason to do anything in the story. It wouldn't have made sense for him and her to get together because he'd just be reminded of the dead girl and she'd feel inferior to a literal ghost, but it would've been a better story and ending than what we got.
So, at the end of the story, what? The kid offs himself to be with the girl? How would that make her happy? How would that make ANYONE happy?
This was one of the most tedious and frustrating things I've read with such a high score and I honestly don't understand why it ranks so high, when the story is so juvenile and poorly written.
I'm surprised people seem to think the MC dies at the end. There's nothing to suggest that he does, other than her saying, "Let's go to heaven together!" But it's obvious she doesn't mean that literally. She's no longer tied to the school so she doesn't have to break up with him when he graduates. The whole point of the wedding dress scene was that even though most people couldn't see her, she wanted him to accept her for the rest of his life. That's their heaven.
Anyway, I really enjoyed this manga, including the middle part that some people consider boring. Remember that she's essentially been alone for 60 years - she needed that interaction with people to grow as a character.