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Who do you think the serial kidnapper is and why?

Whats his obsession with needing a bride?

Even though he is a murderer, it was a botched kidnapping which is why I'm not calling him a serial killer.

I'm replying to myself first and saying that I think it's one of the CEO's brothers.  And I'm not sure why he wants a bride but I'm thinking it has something to do with his face which is why he wears that mask.
The oldest brother does seem very fishy. He could have an inferiority complex considering his father isn't making him the heir despite being the oldest, which could make him go crazy (maybe?). I think the show still needs to progress to figure out the whole bride situation.
Yes I also think it's one of the brothers. Many people are suspecting Bong soon's brother but it can't be because she would have recognized his voice right away at the hospital, also it can't be Jisoo because while he was investigating the masked man appeared on the roof of the building. So I can only think of the oldest brother.
Human experiements
The kind brother with glasses. See I am suspicious of people with zero sass, that's clear a no go.
Yeah it definitely can't be Bong Ki or Ji Soo... the brothers seems like it's one of the most likely options but I also think maybe it was someone we were introduced to right away like one of the cops in Ji Soo's precinct?
I've changed my mind after re-watching the episodes again.  I don't think it's one of the brothers.  One of them is after the CEO but I doubt one of them lives in her neighborhood so I don't think one of them is the kidnapper.  It's definitely someone with a facial problem or someone we should recognize otherwise there's no need for the mask. Plus he's tall for a Korean.
I think he is the witness who saved the "first bride". Some trick! 
He is probably kidnaping because of loneliness
haha he remindes me of Voldemord in that mask ????
I don't think it's the nice brother (is he the oldest?) who's the kidnapper. I'm pretty sure he's the one after Min Hyuk, not kidnapping all the girls... Though the upper portion of his face does look suspiciously like the doctor from that room! (but Bong Soon didn't recognize his voice when she ate with the brothers, so I don't think it's one of them). 
hmmmm... I don't know? I think he looks likve Voldemort too though lol
Although the skin problem idea is somewhat plausible, I still don't think that's the reason why he uses a full-face mask. During the hospital kidnapping scenes, he only covered the bottom half of his face with a surgical mask to simply disguise himself and wore a pair of glasses to impersonate the real doctor. As obvious as it sounds, I think he wears the mask literally to disguise himself and to give a creepy skin-crawling effect of "no face". I don't think there's any other particular deep meaning behind it. As for the older brother, he is very suspicious (and definitely up to no good tbh) but I feel like with kdramas like this, the writers are probably purposely trying to mislead us?? I have a feeling it's just a completely different character with obsessive psychotic intentions.
Based on tonight's ep... especially from the scene where he's treating his wound he seems younger and is obviously super fit which makes me think it's not the oldest brother. I agree, I think he's the one who's targeting Minhyuk but not the kidnapper. I just don't have a good lead on who I think the kidnapper is... it makes this drama that much more interesting
I think the one after the CEO is his nice brother since he is the least obvious, but for the women kidnapper I think is the guy that's at the car junk yard when the police arrived. Bong Soon said he smelled like metal when she passed by him so it would make sense. 
I think that the kidnapper is the boss of the car junk yard..and the nice brother is pretty much announcing that he is after Min Hyuk,to the point that I don't think he is the one,cause that will be too obvious