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.

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.

The kind brother with glasses. See I am suspicious of people with zero sass, that's clear a no go.

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.

He is probably kidnaping because of loneliness
haha he remindes me of Voldemord in that mask ????

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.

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. 

Well at least we know what the kidnapper looks like now but we don't really know much about him other than he's young and fit, and has what looks like a burn scar on his left shoulder.  He probably is related to the junk yard in some way but it looks like he's keeping the girls in an old jail.