Lilli_

Yes, he mentioned it in ep. 1.

Yes, in ep. 1 he mentioned that he's searching for his younger sibling but hasn't found him or her yet.

New theory: Hong Joo is Daniel Lee's lost younger sister (not the adopted sister that was killed by Han Seo Joon but his blood-related sister) and he knows that she was involved with a psychopath, which is why he's so interested in psychopath genes.

I don't think it's possible that Hong Joo is actually a psychopath herself. She was truly afraid of what was happening in ep. 1 when she lured the victim to the killer. That's not how a psychopath would react. And as an adult she seems to feel guilt in quite a lot of different scenes. Again, not an emotion a psychopath would feel.

Even though the respective scenes were shown back to back, they don't necessarily have to have happened at the same time. It's possible that some time might have past which would have allowed the lawyer to return home.

I thought so at first but they could be misleading us with this. Now there's also the possibility that Hong Joo is Daniel Lee's younger sister.

I'll copy my comment from a discussion with another user to explain. It was about Han Seo Joon asking Daniel Lee in ep. 1 if he found his younger sibling:

"I just rewatched the scene to make sure and it turns out we were both right so to say haha. The Korean word used is indeed gender neutral, so we don't know if he's refering to a little brother or a little sister and that's why I automatically assumed that he referred to the little sister's body.
But the following sentence is actually very important and I didn't notice this before when I watched the episode. Daniel Lee says: "Holt is still searching for him/her, but they haven't found him/her yet." So I looked up what Holt is and it's actually an organization that provides adoption services among other things. And we know that Daniel Lee was adopted. So Han Seo Joon indeed didn't refer to the sister's body like I thought but to another character. It seems like Daniel Lee has a blood-related younger sibling and the two of them were adopted into different families.

Since we don't know whether it's a man or a woman, it could actually be Hong Joo if she isn't Detective Park's daughter after all."

I think he referred to the culprit who killed his adopted sister in this case.

I just rewatched the scene to make sure and it turns out we were both right so to say haha. The Korean word used is indeed gender neutral, so we don't know if he's refering to a little brother or a little sister and that's why I automatically assumed that he referred to the little sister's body.
But the following sentence is actually very important and I didn't notice this before when I watched the episode. Daniel Lee says: "Holt is still searching for him/her, but they haven't found him/her yet." So I looked up what Holt is and it's actually an organization that provides adoption services among other things. And we know that Daniel Lee was adopted. So Han Seo Joon indeed didn't refer to the sister's body like I thought but to another character. It seems like Daniel Lee has a blood-related younger sibling and the two of them were adopted into different families.

Since we don't know whether it's a man or a woman, it could actually be Hong Joo if she isn't Detective Park's daughter after all.

Guys, what if Ba Reum was actually right all along and the most recent serial killer wasn't the old detective but the lawyer? He might have known about his son's killings and wanted to protect him out of blind fatherly love. This would also explain why he lied about having killed Detective Park's daughter since he simply didn't know that she wasn't killed as he isn't the culprit.

Furthermore, his way of talking after he got caught seemed way too normal for a psychopath who gets excited when he sees people suffer. One would expect reactions similar to Han Seo Joon's after his identity was revealed. And I also saw a comment mentioning that there seemed to be two people and the one attacking Bong Yi in her home and the guy Ba Reum chased weren't the same person. I personally didn't catch this while watching the episode, but if it's true, if would perfectly fit into this theory.

And last but not least, why did the lawyer behave strangely during the fake interview when he's supposed to be innocent?

You're right, the medicine is another detail speaking for Ba Reum = cross killer, Yo Han = innocent victim.

I don't think there's another character. The Korean word for "little brother" and "little sister" is the same, so you probably just saw a mistranslation.

They stated that the cross killer is the most vicious kind of psychopaths and he's also highly intelligent since Jae Hoon was said to have an IQ over 160. As he's so vicious, I can totally see him not being afraid of getting hurt and thus I think it was all part of his plan. He knew that Bong Yi called for help and that he would be rescued, so he let Yo Han attack him in order to put all the blame on Yo Han. Of course this was a risky move and he could have been really killed, but I think a psychopath wouldn't mind that kind of thrill. Plus, the cross killer is extremely confident and even sees himself as a god, so he might just have ruled out the possibility that he could be killed in this scenario.

And he didn't let his friend alive on purpose, but Yo Han was a genius and thus managed to save him during the surgery. Ba Reum actually kept monitoring his friend by pretending to look after him all the time, so he'd certainly have killed him once he showed signs of waking up. I think he didn't kill him yet while in coma to avoid unnecessary suspicion.

And since Ba Reum now doesn't know that he has to monitor him, I think his friend will play a crucial role in bringing down Ba Reum in the end. I expect him to wake up towards the end of the drama and stand as a witness.

To everyone who finds Ba Reum's aunt and Hong Joo fishy, of course there could be more behind it but there definitely doesn't have to, their actions are easily explainable.
Ba Reum's aunt could simply be reluctant to tell Ba Reum that the bird was killed (I assume she doesn't know it was done by Ba Reum) since she doesn't want to hurt him. And Hong Joo acting strange could be because she feels guilt towards Ba Reum since her lover attacked him.

I had the same thought, there are quite a few parallels to this case. And since the writer got her inspiration for this drama from a real murder case, I could totally see her incorporating other real cases into the drama as well. Maybe there are even more cases in the drama that are similar to real ones and we just didn't realize.

I also think the mouse definitely stands for our main lead aka Lee Seung Gi as Ba Reum, Jae Hoon, the cross killer. However, I don't see the need to weave the transplant theory into here.
We'd usually consider a mouse completely harmless, but when the mouse is being cornered by the snake, we see her fighting back and winning. This is in correspondence with Ba Reum whom everyone thinks of as a weak and naive angel, but in reality he's strong and vicious. But I think the confrontation between the mouse and the snake must have a deeper meaning as well.

As for (probably) young Hong Joo crushing the mouse, I think it could be seen as her will to crush serial killers since the mouse equals Ba Reum, but I can't quite fit her words into this theory yet.

And it's also still unclear to me who could have sent the two mice to Daniel Lee and what they mean.

As you can see, I'm not leaning towards the lab rat theory, but even if we go with this theory, I still don't think the transplant is a necessary element. I'd rather see Ba Reum as the writer's lab rat since she wants to make him repent which is not possible in reality and thus he is her fictional experiment, so to say.

Does anyone have a theory on the symbolic "mouse" and its meaning? I have a few ideas but I can't quite put them together and make it make sense as a whole yet.

There's still the unsolved mystery around who killed Jae Hoon's family, so maybe it was the boxer? He might have somehow found out that Jae Hoon is Han Seo Joon's son and wanted to avenge his sister. But Jae Hoon wasn't very fond of his family anyway, so I'm not sure if this would have made him so furious as to brutally kill the culprit. There must be another reason after all, but nothing else comes to my mind as for now.

Indeed. After he lost his memories he's basically back to the state of a baby and now they "nurture" him into a good guy, while before, people kept telling him that he's different, a monster etc. und thus he indeed became a monster.

Either I missed something or they still couldn't find him. This could be crucial in catching the real cross killer later on.

I don't think they'll go as far as to make Hong Joo an antagonist. But I think she did some bad stuff (probably was forced to in some way) that she can't forgive herself for, which is also why she never returned to her parents.

Well, everyone is different, it's a matter of personal preference in the end, so if you don't care about the repenting part of the story and don't like the side stories either, then maybe this drama just isn't for you. The writer stated that having a psychopath repent is the purpose of the drama, so it's undoubtedly the main storyline. The gene theory was just to set up the story and maybe they'll come back to it again, but it's not in the main focus.
I for my part am still very curious how they'll go about it and how the actor whom I think plays the psychopath that will repent will pull it off.

The main storyline is having a psychopathic serial killer repent his wrongdoings because it's unfair that only the victims have to live in pain for the rest of their lives. That's what the writer said she wants to portray through the drama.