Chapter 1: "his sharp eyes, DESPITE THEIR SIZE, could catch any rat in the darkest of homes."
Wow nyangseob, how rude lmao. Now onto the real stuff:
There were a lot of repeats-especially within the same sentence, it would be best to change that since that actually takes away from the story and your readers already understand what's going on-don't talk down to your readers.
There seems to be a complication with names. When it's just Sunggyu and Ilhoon, you don't need to keep mentioning Ilhoon's name, especially since it's (in a way) from Sunggyu's POV. Also, if it's just Sunggyu, you don't have to keep repeating his name all the time.
Grammar mistakes and spelling errors-but what fanfic doesn't have that? Still, the less the better.
Also, it would help getting more physical descriptions of the characters. Especially with Chaerin and even with Ilhoon, Sunggyu, and Woohyun. Even if the you and reader know what they look like, it still adds more depth to the story and makes it a better read. (Small things count!)
Some parts of the story seemed a bit rushed-like the bakery scene.
Lastly, Sunggyu didn't pay for the hot chocolate the waitress gave them, that's illegal. :T

Chapter 1: Awwww!!! Love it <3