Jo Yi Seo was such a complex character that stole the show whenever she was on screen. It was honestly hard to look at anyone else with the kind of energy she brought to the table. There are things that I didn't like about her character, like her first instinct is to always look out for herself and who she values in a way that may harm others. She doesn't care about collateral damage at all. Initially, she is always intolerant of other beliefs and preferences (relatable for all I'm sure), saying cruel things to anyone who doesn't fit the mold. Which is quite surprising, since she is someone who doesn't fit it at all.
What I do like is, she does apologize and her apologies always appear sincere, even though they might be due to the scolding she has received from the ML. But what can you do, when your mother doesn't care about such things and just cheers you on no matter what you do. Even with her sociopathic tendencies, she had a little heart and she needed someone to guide her. While that be one excuse we can use to defend the character, its just that. Her character was fun to watch and kept things interesting.

Out of all the Park Seo Joon's Drama, this happens to be the one that I'm impressed the most with. It is easy to act all goofy, but the emotions he brought to his characters were simply great. His character was also pretty solid and someone who stuck to his beliefs no matter what. Surprisingly, he was one of the characters that was more open to change as well. What he didn't knew, he learned and wasn't ashamed of learning from someone younger that him.

The Romance:
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There were times when it seemed like it would be natural for Park Sae Roy and Oh Soo Ah to end up together, especially with the way he noticed only her even when Yi Seo was in the room and his stubborn insistence that the he liked Soo Ah, despite anything.

But being with Oh Soo Ah wouldn't have helped him achieve the success & revenge that he was so thirsty for. Without Yi Seo, he probably would have had a very hard time and might have taken additional years to achieve those goals. What Oh Soo Ah achieved through her years of patience is just as important. She was just as lonely as Sae Roy, so in a way, it seemed a bit sad that they didn't end up together. Rather than both of them spending their time apart and alone, they could have worked together, helped each other grow but they each made a choice that separated their paths, ultimately leading to different destinations.

Coming to Yi Seo, she was kind of saint when it came to the romantic aspect. She fell first and everything she did was for Sae Roy (Of course that doesn't negate the satisfactions she received from helping him achieve things no one else could). That is my problem with her romance arc. She was one that was more in love with him. It took him so many years to actually realize what he felt for her. While Yi Seo has sociopathic tendencies, she is not a full on sociopath. She has the capability of understanding & empathizing when taught. So, she must have felt the rejection time and again, especially when he chose to go to Soo Ah. Her pursuit of him was sometimes fun to watch and others just sad. The level of patience she showed for someone so young was commendable. But initially, her feelings were somewhat of a burden to Sae Roy. We did see him reject her, but from his face it was so easy to see he was conflicted and confused whenever she expressed her feelings.
Sae Roy needed to much prodding to come to his senses about his feelings for Yi Seo. Had there been a SML wo could have swayed Yi Seo a little, it could have been much more fun to watch. While Jang Geun Soo tried, he just was an adorable character that seemed a little too young for Yi Seo. Yi Seo was simply someone too old for her age. It was only before she was kidnapped when he was on a date with Sooh Ah that we were finally able to have a clear idea of his feelings for her. Even then, it took a kidnapping.

The worst part about this drama is the the final time skip to 4 years later. They just missed showing us the gradual growth not only regarding the romance but also the development of their brand (IC). It was such a missed opportunity. Even thought they had limited episodes to put that all in, they could have shown us bits and pieces of how things unfolded in those 4 years.