As many other people have mentioned already, this Webtoon is... something, I'll say that.
Insert "spoilers!" warning here. I'll try not to spoil too much, but I still think there's parts that should be mentioned. In all honesty there's a lot of "spoilers" in the story itself because it's predictable as all hell at some points.
Story: K's Secret has a story, that's for certain. It almost feels like there's multiple plot lines that just blend together into one huge story that's a little complicated to understand. A common review you'll see is that the concept itself is rather unoriginal, but I'd say the author did a good job making it unique in it's own way... but then there's also very cliche moments that you'll fully expect from the beginning.
As I said above, some parts are very predictable. Then there are other parts with insane, sudden plot twists that'll send your head spinning in confusion. Things will randomly be dropped on you when you never had any expection of it happening before, and the story will just rush over it then go "oh, yep, that happened! A huge event happened... but anyways, sex." Yep. Best way to sum it up in my opinion.
This story is very, VERY NSFW. It's kind of weird at some parts? Somehow, the author found a line between vanilla smut and BDSM smut and stayed on that line the entire time. It's kinda repetitive but it gets better over time, I suppose. But if you're looking for an uncensored version you'll need like $30. The art near the end makes the payment worth it I guess, but it's not so great at the start.
Art: Arguably, this is the main allure to this Webtoon. The artstyle at first wasn't so great. Something so interesting about reading this is that you see the artist getting better and better at drawing throughout every single chapter. It's one of the things that makes reading this worthwhile. I could see people just using some chapters as oneshots, they won't need much context for the somewhat bland smut scenes. However, if you're reading for more than just the pretty looking smut scenes near the end, you're gonna want to start from chapter 1.
Characters: Cliche. Expected. Somewhat mysterious in ways that are never even explained. As another reviewer mentioned, Jooyoung is probably the best character. He has some actual development throughout the story and in the end readers will find themselves getting attatched to him. I wish we had seen more of him and his story, especially his relationship with Yoon's parents. So much was glossed over and barely explained about his backstory. Sure, he's not the "main character," but I'd argue he's the backbone of this entire series.
Yoon is pretty. I like him for that. But he's also really dense and selfish at times. At some parts I actually was able to pity him which is a pretty good development, I guess. But let me tell you - he barely changes throughout the beginning of the story and the end. Classic uke energy.
As for Kangwoo... I have no idea how to feel about him, honestly. He's a mystery throughout the entire story and I feel like we barely even learned about him. At some parts, he seems very manipulative and even... cold? He's implied to be sadistic in the first few chapters but changes over time into someone who basically only thinks about sex nonstop. Still a fairly average seme.
Overall, I don't know what I just read but I still recommend it if you're bored someday and want to admire pretty art and an "okay" storyline. It probably won't be one of your favorites but it's still worth reading.