While the credits acknowledge that this was "inspired" by Eto Mori's 1999 novel, which was already filmed twice before, the truth is that everything original about this movie - the premise, the idea of a not-so-nice guardian angel, and the final revelation - comes from the novel.
This film presents some really problematic views about women which are not part of the original story, namely how the protagonist treats his mother and girlfriend. The director and screenwriter make it seem that females are to blame if their suffering causes the male protagonist to feel bad about himself. In the original, his motive has more substantial and understandable reasons. In this story, he is utterly selfish. The local reviews upon reviews basically said the same thing, and that the weak script is only carried by the lead's devoted performance.
Watch the 2010 anime version "Colorful" instead, it is superior in every way.
