On a second view, I found very appealing the older sister, I like snarky character. The boss was a sleaze who I despised. I was particularly repulsed by the fact that he tried to act as if his behavior was completely normal, by telling her that if he was not a coward his heart would explode, and pretending that the ask here was to be perfect all the time, rather than, say, to tell his girlfriend in no uncertain terms that he wanted to split up, as he then effortlessly did. The guy repeated this hogwash multiple times.

Another disturbing aspect was the way he acted to his girlfriend as if he didn't cheat because they had split up, but otherwise acknowledged that they didn't to Eun Hee. Also disturbing that the latter and her friend/future lover first joke about her boss' shamelessness, then she agrees to become his mistress, despite having just told him that she had been cheated on in the past, after an almost decade long relationship. She was even accused of cheating, and even has a flashback to the event, asking herself what changed... well, what changes was that at the time you didn't cheat, and this time you did. The scumbag even hid the fact he was in a relationship from her, which ought to have been a deal breaker in and of itself.

Now, the presence of his girlfriend limits the damage as she essentially "blocks" them, but that's besides the point. Her attitude is what it's impossible to understand. She tells her boss that since she was not the reason for the breakup she does not feel burdened. However by the same token the "other woman" she despised and has now turned into could have used the same excuse. Lastly, to break a spear in her favor, I can see why she would break up with her friend, given the latter tells her that her relationship was not guaranteed: that's not why she was complaining, it's not about the relationship being broken, it's about not being lied to for three years and humiliated.

All in all, I don't really like her friend going along with what she does, rather than telling her how effed up it is. Particularly given the parallels between it and what destroyed her family, or her sister's family: the way her behavior mirrors those other situations, or the one with her ex and his "other woman", who are all treated, correctly, as lying scum. Double standard much?

Returning to the boss, it's impossible to understand why one should take seriously him uprooting his entire life and going to Korea, the importance meeting his pen pal had in all this, with the fact he doesn't bother to break up with his girlfriend before that, and deceives his pen pal (no reason, it would have been easier not to do that, and it could have lead to said pen pal not wanting to have anything to do with him anymore, when this situation was so easily avoidable). Also, I would have liked clarification regrading the accusation of him touching his drunken subordinate inappropriately (it's something rather important to disprove, I would guess). And the fact that apparently he had cheated in the past. Not to mention the fact that he had been the one to beg his ex gf to stay with him three years prior, and now he deceives/hurt/humiliates her by cheating on her, and not even having the courage to own up to it.

All in all, I feel that the "evil gf" trope was kind of abused (personally, I like to see the ex gf "counterattacking", despite this not being a LN). She was made to act crazily, while in reality, factually speaking, she had more than enough to complain, what with her boyfriend cheating on her in the past, harrassing her into not leaving him, and then cheating on her right before breaking up?