It might be an unpopular opinion, but the Sooah arc is kind of amazing. Don't get me wrong, this arc doesn't make me feel bad and forgiving for Sooah or anything, she's still a manipulative, self-centered bitch, but the author gives us the reason why she became such a person. You don't need to sympathize, but it's good to understand. And, the way it approaches the main theme of this series - how beauty obsession can lead people the wrong way - is interesting. Because everyone comes to like her just because her beauty, she consider natural beauty as her key to happiness, and having all the guys like her as the proof that her beauty is enough to keep everyone around her. And everyone around her just falls into her trap, keep proving that her way is effective, that's why she keeps falling into it and get more and more greedy. I can see the way how her mind become twisted quite clearly.
And, the story about the girl whose face she can't remember is interesting. She's the only one that Sooah doesn't value by face, but actually remember what she said, remember that she wanted to be her friend. She could have been the chance for Sooah to quit her beauty obsession and start value people as their own personality, but Sooah missed that chance because she can't get in touch with her feelings. But this time, with Mirae, she might get that chance again, because Mirae is more determined in helping her, and because she had shown Mirae her true ugly self, not the fake nice one. It's a good development, if you ask me.