Note: This is solely my opinion, and yes, I'm not that good at writing. Sorry if it's kinda redundant or doesn't make sense at all.

STORY:
Okay, starting with the story/plot of the series. Nowadays, cliches are quite common (this sounds funny tbh). But I can say for this drama— it has quite a fresh approach in some parts of the plot, it got me hooked with the first episodes after all.
Starting with the romance side; the chemistry was good, the cold vibe to warm between them was typical of course, but the thing is the way they "got to the warm era" was a little fast and subtle (but for me tho, I think I would prefer the cold vibe hehehe).
On the thriller and mystery side, it was nice, it was what kept me watching. It had a bunch of mini plot twists, the first twists were good, but the coming to an end twists weren't: like the fact that it was his father (real real father: the chairman) was the one who arranged the accident was something you can just guess and the fact that he was in Argan😭, I think it would have been better if they put more characters so that the audience can take more hunches of who's behind it— just like how I had my own theories wether Ji Sang-Woo (Hee-joo's sunbae) and the guy (the real Sa eon) were twins, or the two of them and Park Do jae were working together (Sang woo was kinda sus, yeah), it's usually in dramas where they put characters to make your brain work for hunches and theories. And I can say WTPR executed that part well, also the contract between their father and the chairman, and the best ones were: she's not mute, and it was him who requested for Hee-joo to be his wife— the worst was the accident part. If there was a greater twist than just using the same when the guy whispered to Sa-Eon it would've been better (but sometimes too much can be too much indeed, unless with great execution). But they did well and much better on the mysterious side of the plot rather than romance. I also don't like the part where Sa-Eon was at "Argan," although they mentioned why, it still feels far from the plot, it felt as if they just created it clumsily to find a reason why (but I know it was also part of building the romance and more tension), it would have been better for me if the plot just killed him or somebody at least hehehehh (but I guess, either way would've disappointed me).
At In-ah's arrival, I thought at first that she would be that mean, jealous sister, but glad she isn't.
Right. One thing I don't like about the deal/contract of the chairman and her father was that the agreement/content was quite weak.

CAST& ACTING:
The casts we're great with acting. Sa-eon is a great character executed well by the actor, and Hee-jo as well, although she was good at it, I think I would also love to see other actresses try her role, it's like she fitted into Cinderella's shoes but had a little more space left.
The in-laws were good too, their characters' personality were delivered clearly.
Their friends and colleagues, too. But I think Kang yeong-u and the spokesperson needed a deeper chemistry.
Park Do-jae was a good character, but I think he just needs a little more work.

CINEMATOGRAPHY&SCENE IMPACT:
The cinematography was great, the scenes as well, it really brought the mysterious and thriller vibes. Romantic vibe as well, but kinda failed a little, for me.

OST:
The OST's greattttt. See the Light nananantantananneunnnn. (Especially since it's BTOB's Eunsik!!!

FURTHER COMMENT:
To be honest, I fell off the drama. I couldn't help watching the first two episodes, so I ended up waiting for DAYSS AND WEEKS which was sad. That's why you should binge this kind of drama. So that the thrill doesn't get cut off. Each episodes ending were a cliff hanger which is good for attracting viewers into finishing it.
The ending was nice and warm, but what happened before it doesn't really fit the plot well, I mean it was actually so random that Hee-joo goes to a war zone to find him, and there he is rescuing hostages. I also think that the plot just needed a deeper relationship with each twists and scenes, and the execution can be better.

OVERALL:
It's a good drama for beginners, since I think I kinda dislike it because I've watched a whole lot of kdrama and is looking for a good one, but my definition of "good" is kinda hard to fulfill. Yeah overall, it just needs a little work and some changes. But good enough to please viewers that doesn't really think deeper about dramas, or just looking for something to use their time with.

P.S: This type of drama is the one that kinda fails a little when reaching the end. In other words, it's going on a descending order in terms of interest.