This is going to be more of a review and my analysis then a comment.
The story starts off as if Se-Wa is the crazy one in love with her brother, when actually it's really her brother, Se-Joon, who is obsessed.
We find out that his obsessive love started when they were children, when Se-Wa was seperated from Se-Joon for the first time because of her heart transplant. It was then that he realized how much he loved. So, he kept her isolated from everyone by spreading rumors about her and doesn't help her when she's bullied because he wants her to feel like she only has him by her side, which he does so successfully.
Se-Wa becomes totally dependent on her brother's love, since she has no one else. She grows attached to him and constantly seeks his love. In a sense, Se-Joon completed what he desired, which was to have Se-Wa's affections just for himself. However, his guilt and his fears of what the world thinks of his love for his sister causes him to push her away as if her affections for him are the cause of his misery. He puts all the blame on his sister, as if she's the one obsessed with him, as if her love for him is suffocating him.
Se-Wa eventually tries to move on from her brother. She finally makes friends with Gi-Hoon, and slowly she falls in love, for the first time, with him. We learn that her love for Se-Joon is nothing more than a child's attachment, seeking for love. However, she doesn't refuse her brother when he tries to sleep with her because she was still confused over her feelings for her brother and what they meant. When she sees Gi-Hoon for the first time after sleeping with her brother, she realizes the difference and her feelings for Gi-Hoon that were developing.
After seeing Se-Wa grow apart from him, Se-Joon realizes what he has done. He becomes possessive again, and starts hurting people who has tried to "dirty" or hurt his sister. As she grows farther and farther away, his desires grow that he even starts to hate his sister and wants to kill her.
Gi-Hoon, who initially wanted revenge, falls in love with Se-Wa, who has his younger sister's heart, which was stolen because of Se-Wa's father. Gi-Hoon soon finds out that Se-Joon slept with Se-Wa. Se-Joon panics and continuously beats Gi-Hoon unconscious with a rock. Thinking that he had killed Gi-Hoon, Se-Joon runs away, bringing Se-Wa with him.
Since Se-Wa’s heart isn’t really her own, she constantly has to take her pills. However, because of Se-Joon’s selfishness and fears for himself. He actually neglects this fact, and Se-Wa’s heart condition worsens and eventually dies.
Gi-Hoon blames Se-Joon, and Se-Joon knows that it was his own fault. He goes to find Se-Wa’s cell phone and gives it to Gi-Hoon, saying that the phone belongs to Gi-Hoon. Gi-Hoon reads Se-Wa’s message to him and cries. I have no idea what it says exactly, since it isn’t translated, but it’s probably “I love you” or something. Se-Joon believing that he can’t live without Se-Wa, commits suicide.
In the end, I think the father got back what he deserved and loss both of his children even though he sacrificed Gi-Hoon’s sister to save Se-Wa.
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Also, I'll like to comment on how the story began and ended with the story about the boy who everyone loved and whether you would choose to be loved by everyone or just one person. The boy was like Se-Joon, but even though he said at the end of the manga that he rather by loved by one, his action throughout the manga was contradictory, since he was so afraid of exposing his incestuous relationship, afraid of what "everyone" would have thought. So, to me he choose to be loved by "everyone."