I have quite clear in my mind what I liked and what I didn't so here it goes .
I think I liked Da On's storyline the best. The character we get presented is super clear and well made, and that's followed by staying coherent in every decision he makes in the story, but my probably only complain is that I felt that he fell a little flat toward the end (he like, did almost nothing for 2-3 episodes) and it felt like it was on purpose to elevate a little the other relationship.
Shin Woo is also a great character, and made me angry, happy and also sad, he also made sense to me so I enjoyed watching him lots. I didn't love of him that the only explanation we get of him being mean to TK is that he's awkward and shy, didn't feel like enough.
Both this actors did amazing jobs with their characters and I loved to see that.
Tae Kyung is nice to follow, also very coherent and I love how direct he is with his thoughts, but it did feel like the character was a little flat, so I didn't get to develop a deep conexion with him (like I did with Da On), and added to that the acting wasn't that impressive so that's that.
I didn't feel romantic chemistry between TK and SW at all, they always gave me more of a friend's vibe, so I wasn't quite satisfied with the ending (even tho it made sense, so I'm thinking it's more a thing of the actors lacking that chemistry). I did feel chemistry between TK and DO so that's that.
Namgung was super fun and wise but didn't have much to offer so that was kinda sad.
So Hee was, imo, a badly made character, she's the only girl and she's the bad jealous not girlfriend, she doesn't offer anything good to the story, and she proceeds to do something awful (out someone) and then it's all fixed super cleanly and at the end they're all friends like it's nothing. It's depressing that she's the only female of the story.
The teacher was cute and fun and wise and his participation was enough and well thought. I'll be looking forward to a series for him! That ending got me excited.