Title [4/5]
For the most part, the title does make sense, but it isn’t quite right. It would have been more appropriate as Paper Planes and Tinted Windows, or just Paper Planes as you’ve named the chapters, because all the events occurring have something to do with Chanyeol’s planes more than they have to with his neighbor having tinted windows, I believe (I took a peek at the sequel, and it supports my justification). The title was as original as it can get in a world like this where nothing can be done without some reference to something else, and I commend you on that.

Description & Foreword [10/10]
This is probably where most authors do not know quite what to do; yet you managed to let the readers know the gist of the story without giving too much away. When I read it, I immediately understood that Chanyeol had a secretive neighbor whom he was trying to contact, and that the story would focus on the mishaps that happen because of the planes he flew.

Plot [19/20]

There were some cliché elements (mostly the fact that Yifan and Chanyeol got together in the end, but that is logical) as there were some original ones as well (an anonymous neighbor AND pen pal; it’s usually either one of these instead of both together).
I particularly enjoyed the twist, where his sister reveals Luhan is possibly their neighbor. I had convinced myself that Yifan was the neighbor, so his sister seeing Luhan and Jongdae seeing Yifan definitely gave some food for thought.

I’ll leave you a few questions, though. What would have happened if Yifan had truly become marred in the fire? And what about the tutor we meet in the beginning? What if he had a bigger role in shaping the story?

Flow [15/15]
It was rather slow at first, but it sped up nicely and kept going at a (mostly) smooth, even pace before ‘tumbling over’, as I say most nice stories do.

Characterization [13/15]
The development of the main character (Chanyeol) was very believable, and it was a pleasure to know of his feelings so comprehensively. However, the supporting characters were rather 2D and ‘flat’. In my opinion, it would have been better if Luhan and the others had been given a more 3D composition, since they play a major part as supporting characters. Normally, I would have said something about Yifan as well, but since his personality is mostly mysterious, not to mention his identity is ‘unknown’ until just about the end, it seems sensible that not much of his characteristics would be known

Grammar [19/20]
I docked a point here, because there was some inconsistency with tenses. These were somewhat rare, though, and unnoticeable for the most part. The spelling was good, and the story was fully understandable grammar-wise.

Readability [3/5]
The font size was a bit too small, but other than that, it suited the story well. I know that it may have seemed too long if the font size was larger, but that can easily be remedied with fewer lines in between paragraphs and/or less spacing between them.

Personal Enjoyment [13/15]
Honestly, I’ve been of two minds throughout the story. On one hand, it had been an interesting read, but on the other hand, I personally find romanization irritating. I understand that it was essential to your story, as without it, it would take more words to show how Yifan distanced Chanyeol, for example. Instead, you did so by having Yifan call him ‘Chanyeol-ssi’. Moreover, there was the issue of an author’s note in each chapter. The readers would like to know why it took so long to update, yes, but maybe you can limit it. Overall, despite these little issues, you have good skill as a writer. You know how to keep the suspense up throughout, and your descriptions are apt and to-the-point. I look forward to reading more of your stories, and I hope my review was helpful.