Abel has deep prejudices against Ji Yeonli (also Beth, he thinks Beth just a mere woman, he doesn't understand why would Cain fall in love with her) , not only because she killed his brother, but also because he knows and has heard about the things she has done to Park Haena. Therefore, he honestly believes that Ji Yeonli is nothing but an evil woman. But what happens if he discovers that nothing is as it seems? What if Ji Yeonli is actually a very sweet and caring person and she did what she did for a reason? I'm very sure he will fall madly in love with her. The more he hates her, the more he loves her.
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If love is the central theme of this story and if Abel and Ji Yeonli are meant to be, then the author is likely to choose to have Abel and Ji Yeonli's first time occur when they both genuinely develop feelings for each other (referring to actually doing it, no unexpected turn or cliffhanger like their previous sex scenes). This would emphasizing the contrast to all the previous sex scenes throughout the chapters (up to chapter 50) that were driven by desires (Park Haena x unknown celebrity, Park Haena x Her Fiance, those cult members, Abel x Park Haena) and one-sided love (Cain x Ji Yeonli, Ji Yeonli x No. 17).
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Points to support my assumptions about Abel and Ji Yeonli ending up together?

Ji Yeonli is literally broken. She has been through a lot and has serious mental health issues. Cain has been by Ji Yeonli's side for years (at least 20 years) since she was a little girl, but Cain is still unable to fix her. Therefore, I think the author will choose to explore Ji Yeonli's journey towards healing and happiness without the presence of Cain because it seems like Cain is unable to provide the support she truly needs.

This is my perspective after knowing their story. I also suspect that Cain has been searching for Beth over the centuries, and he recognized Ji Yeonli as the reincarnation of Beth. That could explain Cain's strong attachment and protective nature towards her.

As the story progresses, I noticed that Abel and Ji Yeonli are quite similar to each other. And their similarities make Abel more likely to be the one to heal and grow together with Ji Yeonli.
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i. Both of them didn't really know anything about love. Yes, Ji Yeonli also didn't know what love actually is, just like Abel. Both of them have a really weird perception about love. For Ji Yeonli, she thinks caring is the definition of love (When she says she loves Cain, I think she actually means she cares about Cain). And for Abel, he thinks sex or intimacy is the definition of love. Is there anything more interesting than seeing two fools who know nothing about love fall in love with each other, embracing healing together? And maybe because of this reason, the way they act when seeing the one they care about get into trouble is quite similar. They do what they think is best for the scenario without putting themselves in the shoes of the person they care about. Just like how Abel forced Cain to stay with him alive, even though Cain wanted to die after he witnessed Beth's death. And how Ji Yeonli tried to drag Park Haena out of danger by shutting her out in the cruelest way (I'm pretty sure Ji Yeonli did what she did to protect Park Haena, and Park Haena's child, who is a little boy or girl in the orphanage).
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ii. Both of them are struggling with self-acceptance problems. As Ji Yeonli mentioned (in chapter 37), Abel is a perfectionist. He can't tolerate any mistakes or imperfections. That's why when he realized his emotions were being controlled by the golden bullet, he's gone mad. He cannot accept this, and this is actually the reason he hates Ji Yeonli the most, more than the fact that she killed Cain. This is obviously caused by his self-acceptance problems. He is kind of mindful of the fact that he only has one wing (unlike Cain, who doesn't mind at all). He cares about how the other angels think of him. That's why he tried so hard in heaven to do everything perfectly because he wants to be the archangel and impress every angel, letting them know that even though he has just one wing, he can still do better than any of them. But deep down, he still does mind that, and getting a pair of wings is one of his three wishes. The Queen of Hell promised to grant his wishes if he successfully catches Ji Yeonli alive and takes her back to hell (I think the Queen of Hell is actually the true villain). Ji Yeonli is also struggling with self-acceptance problems. Remember that she told Abel that she attempted to end her life so many times just because of the voices from the audiences, her fans, and everyone. Her emotions go up and down based on people's voices. It's really awful to live like this. Ji Yeonli and Abel both seek validation from others, caring about what people think of and judge them.
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iii. Both of them want to be loved and feel loved. Isn't this interesting? Abel and Ji Yeonli, who know nothing about love, desire to be loved. They share the same deepest desires. Both of them have been through a lot, leading them to need a lot of love to heal their hearts. Both of them cover it up with their shown images. Abel hides his vulnerability under his cold facade, treating people coldly and distantly. Ji Yeonli has hidden her vulnerability under her sweet face and personality in the past; everyone thought she was living happily like a princess, but nobody knows she is actually broken. Now she covers up her vulnerability under her strong and powerful image; only those who work with her know Ji Yeonli can't control her emotions if she doesn't take pills.

Remember the conversation between Abel and Ji Yeonli when he was having hallucinations (chapter 30)? He thinks nobody loves him, not even his brother, because he thinks Cain always puts someone else before him. The first time he chose Beth, and the second time he chose Ji Yeonli. That's why he feels unloved and abandoned.

It's clear that Abel's deepest desire is to be loved. That's the reason why Abel is able to calm down and defeat the effects of the golden bullet whenever Park Haena says she loves him. He was able to calm down and accept her kisses not because he has developed feelings for her (he even asked Park Haena to stay away from him when she held his arm), but simply because he wants to be loved and feel loved, especially at that moment when he was feeling helpless and certain he might lose control to the golden bullet again. Abel's misconception about love has clouded his perception. He believes that love can only be expressed through physical connection, which led him to confuse his lack of disgust with feelings of affection towards Park Haena. Since he knows nothing about love, I believe Abel didn't even understand what was exactly going through his mind; he didn't know he was actually longing to be loved.

I would also like to point out the fact that Abel didn't develop any feelings for Park Haena during their 3-year contract period. Why would Abel suddenly fall in love with her? Just because he had sex with her? Do you really think it's possible for someone as cold as Abel, who didn't fall in love with Park Haena during the 3-year contract period, to suddenly develop feelings/fall in love just by having sex with her for 4 days? Not to mention that Abel can't stop thinking about Ji Yeonli (he claimed it himself in chapter 43) when he was having sex with Park Haena and also the fact that he doesn't even know what love is.

From his understanding, love = ecstasy and ecstasy = sex, he didn't even know there's a difference between love and pleasure. And the most important thing is that Abel had sex with Park Haena not out of love or any special feelings for her; Abel obviously just used her as a tool (Park Haena is the one who initiated). I actually quite understand why the author let Abel lose his virginity to Park Haena. If Abel didn't do it once with someone he doesn't love, how is he going to tell the difference between the feeling of making love or having sex with someone he loves or doesn't love. Sex is a part of love, but SEX ≠ LOVE. Abel has never loved before, and now Abel has been shot by the golden bullet, so he is definitely not going to fall for Park Haena.

And I noticed Park Haena's reaction is quite interesting whenever something happens between Abel and Ji Yeonli (hints at Abel and Ji Yeonli's romantic development). She gets frustrated when Abel tells her Ji Yeonli shot him with the golden bullet (chapter 24). She believes Abel might actually be attracted to Ji Yeonli, which is why he lost control on that day (chapter 24). She looked anxious when Abel shot Ji Yeonli with the golden bullet (chapter 41); maybe deep down, she knows their connection will eventually lead to love, even if it's influenced by the bullet's effects. She refused to help Abel learn the ways to love (chapter 45). She gets jealous and claims that Abel might fall in love with Ji Yeonli (chapter 45). And the way Abel didn't really care about her feelings after she had gone mad already tells us Abel didn't develop any feelings for her. The only thing Abel worries about is losing control like Park Haena did.
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Some thoughts about Ji Yeonli:
I am quite certain that Ji Yeonli hasn't changed. She is still the sweet girl that everyone thought she was. Perhaps she is acting like an evil woman to conceal her true nature and make the devils believe she is strong enough to defend against them. The author has been giving hints about her true personality throughout the story, specifically in chapters 8, 10, 19, and 33, which mentioned how she was/is acting before/during these events. I am quite certain that what she is doing now is for some holy purposes.

In my opinion, the main purpose Ji Yeonli wants to achieve is to bring Cain back to life. That's why she kills the devils to collect Ichor. As I mentioned earlier, I believe Ji Yeonli might give the heart back to Cain in the ending because Cain's power has now transferred to her. Additionally, I think there are missing pieces of that day when Ji Yeonli killed Cain. I believe something must have happened between them, which is why the author only shows us the pieces before and after Cain's death. A wild guess, I don't think Ji Yeonli was aiming gun at Cain. Instead, I think she was aiming the gun at herself. Perhaps at that moment, she wanted to die without fear, to experience the moment where she could only feel love for Cain. That's why she attempted to shoot herself with the golden bullet.

Another purpose is to destroy the hell kingdom. I'm not entirely certain of the reason, but I believe it's related to Cain and Abel. Maybe Cain discovered that there is no way they can return to heaven unless Abel completes the quest of 'saving a dead human by helping that person get to heaven.' However, since they made a deal with the queen of hell, Abel has been focused on making contracts with humans and collecting souls. He hasn't made any effort to complete the quest assigned to him. As the number of people making contracts with devils increases, the likelihood of Abel completing the quest decreases. This is because those who engage with devils will not be accepted into heaven. I believe Cain mentioned this to Ji Yeonli before, and she knows how much Cain loves Abel. That's why she is attempting to do this for Cain.

And one last thing, it can be seen that Ji Yeonli is helping those who have made contracts with the devil by exchanging things with them, such as money or any valuable items. In my opinion, she is trying to earn money and acquire useful resources from those wealthy individuals who ask her to kill the devil in order to obtain enough funds and resources to eliminate those disgusting high-profile and rich people (all those people associated with Park Haena's ex-fiance).