Well, ep 6 has clarified the appearance of the serial kidnapper. Safe to presume that it's a character who has yet to be formally introduced. He may or may not be acquainted with all his victims prior to kidnapping. The pharmacy scene was a little confusing for me because the dude looked very old-fashioned/retro so I thought it was a misplaced flashback of some sort LOL. And it might just be me but when he took off his mask, something seemed off about his face? Hard to put a finger on it but it might just be due to the lighting and my overanalysing LOL.

Although the culprit's face is revealed, it just doesn't seem to fit. What if, he has an accomplice to take care of 'outside' matters?

The culprit and the fake doctor are probably two different people. The fake doctor has a smooth face while the culprit has a rough face. During the hospital kidnapping scene, the culprit must've already started the car and waited for the victim. There's no other explanation as to how quickly they escaped. It would've taken more time if the fake doctor had to put the victim in the car, then move to the front seat and start the car and drive away.

Sit back folks, for this might be long.

The Possible Accomplice of Culprit:  



Take a note of his face complexion, not-so-full pink lips, dark hair and shoulders.

A full-length pic:  



Take note of his shoes and how his right hand appears pinkish/reddish.

The culprit disguised as the doctor:  



Take note of his spectacles, dark hair and thin eyebrows.

Picture with hand:  



Right hand appears reddish/pinkish.

Another full-length pic:  



Take note of his shoes, which are different from the ones he wears after, showing that has more than one pair of shoes and that's why the footprints were different in both cases, or, they really are different people (who are accomplices).

For reference, the real doctor:



The culprit did a pretty good job of disguising as him, except, their lips look different.

And now, here's a picture of Hyuk's elder brother whom he 'trusts':



Light complexion, not-so-full pink lips, dark hair, pinkish right hand and similar shoulders to that of the 'accomplice'. And if you look close enough, you'll notice that he has thin eyebrows too.

Coincidence? I think not.

Wait, the culprit is the "witness" who reported the second kidnapping attempt isn't he? When he took off his mask, I thought that was him. As Min Hyuk figured out, he didn't mean to kill the first victim so I think when he realized the second victim was badly hurt, he reported it so she would get treatment and to also effectively take himself off the suspect list. He stayed around and reported the crime to avert suspicion from himself but really he was the one who did it. There was no witness and he kidnapped the girl again after she was no longer critical and before she woke up. Plus that's what he meant when he told her he went through so much trouble to "save" her. 

At least we have a glimpse now of why he's doing it, he thinks he's Blue Beard.  He must have been an aspiring actor at some point and an accident happened, otherwise why would he even know about that small play.  And also know where and how to steal the actors shoes.  I also wonder if this guy is gonna end up being the main villain.  I know the gangster boos appears to be, but he doesn't really fit the profile for a typical superhero story.

Yes, seems that he is fascinated by it and it's quite interesting to see that he is not so random stupid crazy kidnapper but kind of intellectual guy? O.O

some weird fetishes out there man lol.