They frame the story as a murder mystery and dedicate a significant portion of their time to a childhood romance subplot. Ultimately, the murder mystery isn't much of a mystery and the subplot takes a turn for the worst because the villain conveniently puts the hero in a coma for 20 years.
Honestly, what killed the story for me is the coma. It's a Pyrrhic victory that undermines most of the effort put forward to catch the criminal.
And don't get me wrong, there is a lot of potential behind the idea. It's an interesting way to resolve Satoru's ability to travel back in time. But something about the execution of it all leaves something to be desired.
The biggest problem with Airi as a character unfortunately is that she isn't a vital ingredient to the story moving forward. You can omit most everything about her character and Satoru will still fight for a better future. I would forget that Airi was a part of the story if it wasn't for her ending up with Satoru at series end.