I will start by saying that this is what "The Fault in Our Stars" should have been. Yeah, yeah, they don't treat the exact same subject, but the difference between Kimi no Suizou wo tabetai versus TFOS is that one of them reads more as a kind of suffering porn (idk the exact term, it had one but forgot it), while the other one feels more like a gut wrenching story about finding yourself through someone else and then tragically and unexpectedly losing them. They seem like the same thing, but one is executed pretty well.
KimiSui has a pretty interesting story. It is sort of bland at its surface, but the concept is interpreted and delivered, and I like that the characters and their personalities didn't overshadow it. And I think that this is the beauty of this movie. It is not about the characters. It is not about how exciting they are, it is not about what they go through. It is about what their relationship means, it is about how their opposite traits blend into each other and are probably the same. A beautiful movie, even though it had pretty mediocre characters and story.