So, the initial premise was actually quite good: People in each world are connected in some way, so what happens in one world affects the ones they are connected to in the other. So when Kotona, a girl in our world, is about to die, it seems like the only way to save her is by killing Astrid, a princess in the other world. So the two main characters, Yu and Haru, go against each other to defend the girl they fell in love with.
The first act is pretty solid, introducing the cast, the two worlds and apparently the main conflict of the story set in trailers and whatnot. The problem starts when the second act begins and things escalate way too fast. In less than ten minutes of screentime, Haru and Yu go from best friends to enemies and go to freacking war against each other. It felt extremely rushed and poorly paced and really hard to believe those seemingly best friends could go to war with each other.
Things only get worse when the final act rolls around and not only we get a giant exposition dumped into the audience to explain the true villain of the story and why everything was happening in the first place, but we also get a lackluster final battle and resolution to everything.
Overall, the wasted potential of a more gray story where the protagonists must fight each other for a believable reason is lost, replaced with a very cliched and overuses narrative.
The 2D art and animation are overall decent enough, with the addition of some really bad and distracting 3D animation thrown every now and again. The biggest problem comes when characters have to move around, like in fights, which are poorly made and animated and very uninteresting to watch. Characters also felt very stiff and showed little to no emotion, either by facial expressions or body language, so they ended up looking like clay dolls with very poor movements.
The sound design and music do their jobs, but there's nothing in there that catches your attention either. You will probably forget all the songs very quickly and sometimes even forget they are playing. As for the sound design, they are decent and do their job well enough, so no real complains there.
There are no interesting characters here, either because they have absolutely no personality, flaws, conflicts or are just plain stupid.
Yu is a really generic and boring character, having nothing interesting aside from being unable to walk. And even that is not used fully, as upon arriving into the other world and finding out he can walk normally, he doesn't react at all to it, despite never having walked for as long as he could remember. If you watched Avatar (the giant smurfs one), you'll remember how Jake reacted when he, a disabled guy, first walks with his Avatar. And he hadn't spent his whole life unable to walk around.
Haru is just plain stupid and I can't say anything else without giving spoilers. Kotona does nothing aside from almost dying, so we don't get to spend much time with her. Astrid is the only somewhat interesting character, yet she does pretty much nothing over the course of the movie.
And as for the villain, he is really bad, only being introduced for real almost at the end, despite some very weak foreshadow to who he was. His motivation for being the villain felt weak and uninspired and came out of nowhere as well, so more negative points.
As I said before, the initial premise the movie hooks you with, two friends fighting each other for different beliefs, is a really strong one. Yet the execution of it is really bad and useless at the end when the third act begins.
And this is more personal, but I hate it when the story lacks internal logic. Like, why wear armor if a sword can still kill you as if you didn't have it in the first place? Why make walls to protect you kingdom when someone can get a bloody rhinoceros-like creature to smash it without any trouble? These and many other moments take away my enjoyment way too much and become distracting at times.
In short, this movie is a piece of hot garbage like no other I've ever seen before. It's less due to being a bad movie (which it totally is), but rather thanks to the utter disappointment I felt while watching it thanks to the very strong premise it makes you think you'll get.