First off the opening is great, it stands out, it's different from the usual energetic openings you'd be used to. It's slow and nice, it's really fitting for the anime.
On the surface it's a usual high school romance, but it's done in a unique way, the main couple form a relationship in the early episodes and it's more about them growing as a couple. At first it didn't need tropes to keep going and stay interesting, the anime touched on sensitive topics and was pretty emotional and open. There was no dragging, no misunderstandings, no bullshit that you'd expect from any romance anime. It wasn't until ep7-8 that the story needed new cliché characters and tropes that challenge their relationship which felt fabricated and a downgrade to the story up until that point. In hindsight the anime could've stayed interesting without having to utilize such things, there was no need to introduce new characters just for the sake of triggering the underlying problems with their relationship, although the growth of these added characters was nice to see.
Also episode 13 is basically an OVA, not only it didn't happen in the manga, it just doesn't add much to the story. Episode 12 is the real closure to the anime, but even that is a bit lackluster.
All in all it's still a great romance story, it starts off really strong, but falls off a bit towards the end. One of those shows that you will love but will always remember as the anime that could've been even better
