I am back! I read quite a few until chapter 4, and I understand your concern about the hook. In my perspective, I found some points which I experienced. I am trying my best to read in one go, just like a new reader, to deliver feedback about the hook. Again, this is what I think, and this may be different for others.
1) I noticed inconsistencies in using "." and "," for character's words. I think it's better to be consistent (I don't really know English typing rules, to be honest, but I always use "," and not ".". What I meant is shown below as example:
"Sure yeobo," Da-som replies.
or
"Sure yeobo." Da-som replies.
Still about typing style, somehow I feel there is lack of conjuctions in between actions when your episode focuses on action/present situations of the characters. For example, in your first episode, in this paragraph:
"Sure yeobo," Da-som replies, then turns toward the youngest. "Want to play a song Hye-jina?" she asks in babytalk.
May be written like this:
"Sure, yeobo," Da-som replies, then turns toward the youngest and asks in babytalk, "Want to play a song, Hye-jina?"
Personally I feel there is no need to break the sequence into two sentences, but again, this is just a suggestion which may be heavily influenced by my own preference.
2) About Korean names, I know the addition of "-ah" or "-ya" is like a sweet nickname to Koreans, but this may be confusing to readers who are not too familiar with Korean terms. I am a bit confused when Seul-ki being called as Seul-kiya at first, then I realized the intention of "-ya" addition. Maybe, this is just me getting a bit unfamiliar due to not watching K-dramas lately.
3) About Korean word usage. I saw "yeobo", means honey (when husband calls his wife and vice versa). I understand it even without explanation because I know the term, and even if Korean culture is in its peak right now, I think at least an explanation has to be given when the term is first appeared. These are my suggestions just in case your reader is not too familiar with Korean terms and language. There's no real guarantee the one who interested in your novel actually someone is too familiar with Korean culture. Also, in chapter 4, Seul-ki's mother did not call her husband "yeobo", but just a simple "Husband". I don't know if this has hidden meaning, but I think it's better to be consistent when using Korean terms to not confuse the reader.
4) Somehow I feel it is not because of 'serious' or heavy. I am quite confused to articulate this, to be honest. I am a serious and heavy stories enjoyer (in fact, 9/10 of my libraries are heavy story). I love the theme of your story. But, I do feel lacking of suspense and tension which I expected after reading second chapter. To be honest, Ha-neul is not that interesting for me, and I got more interested in Seul-ki. Sometimes I forgot the main character is Ha-neul (I'm sorry ). Maybe this is the 'low hook' that become your concern. Emphasizing the tension about Seul-ki wants to go to South may improve the hook (i.e. how he hates the government or a bit picture how repressing the government might help, such as consequences if they got caught that they are catholic?). Another suggestion, maybe adding how Ha-neul actually feels about the situation also helps to shifts focus from Seul-ki, because all I see about her is that she is pretty and that shitty administrator is chasing her.
5) I have this question when I read Ji Yong-beom entered Ha-neul's house in chapter "Catastrophe". It seems a bit strange for me that he is so easily went in. Why the house was not locked when nobody is inside? Or is there any rule in North that the house should not be locked? This seems off for me, but an explanation might be able to solve this one.
Overall, I like your story's concept. To be honest, I read the synopsis after I finished chapter 4 and not expected that the main character should be Ha-neul (but I do suspect a bit since I remembered the cover is a girl), so I have to rewrite point 4 lol. Your synopsis is actually the one hooking. From the synopsis, your story has a lot of potential. The chronological concepts of scenes is good, and at least, I know I want you to screw that administrator real bad. I really want him to actually fall when he got out from Ha-neul's house, specially when going down back to the school. That's actually hooking, though.
P.S.: Wait how come this is so long I am sorry