A girldfriend AND a sister?

the sister and girlfriend are two completely different people.

It's preety unclear up until they kidnap the little girl.
There's a scene in front of some blocks of apartments. Then you can clearly see Ryu, his sister - she's the one sitting on a swing, and Cha Yeong-mi, his girlfriend - the one that's dancing around singing about defeating the communists, or something like it. So, there's two of them...

Soloution: Watch more East Asian movies. "They" might all look alike to start with, but after a you've seen a handfull "they" start to become individuals.

How can this be prevented anyways, considering that South Korea is where it's all happening at the moment, regarding movies.

My brother thought the exact same thing. I never got the two confused. But, I wasn't sure if Ryu's girlfriend was a man or woman at the beginning of the movie. When she's siting behind the computer, you only see the top half of her face.

Even though I'm a Korean, I made the same mistake also, :-D
Yes. There are a girlfriend and a sister.
Cha Yeong-mi is the girfriend who persuaded Ryu to kidnap a child,
but Ryu's sister didn't even know about that kidnapping.
Remember when a woman writes down her phone number and gives the paper to
the kidnapped child? That's Ryu's sister. She didn't know the child was
kidnapped, but thought she was one of Ryu's friend's daughter left in care of
Ryu because of her father's circumtances.

Such a late answer, but want it to help you to understand this film.
This is my first CW Park's film(I'm too young to watch his film) but I think
his other films would be all great as well...

I thought the sister and his GF looked alike. But I was watching it with someone else, and both of us figured out at the same time quite a ways into the movie that they were 2 people.

I saw Oldboy first, and the first sitting was unforgettable. However, since finding out the "twist", subsequent viewings are OK only. A bit similar to how I feel about The Sixth Sense. The events seem a little contrived IMHO.

I've only seen Mr Vengeance once. My impression is that it is more raw, and there seems to be more going on. More tragic too.

for western viewers Asians all look very similar because western people have other things they automatically "register" by viewing at a face. Like for example western faces are more different because of different noses - so you learn from your childhood on that you have to distinguish people by noses. If Asians all got a very similar nose and you should really catch something else (like the eyes or whatever) it begins to be hard for you to seperate people from each other.

nose and eye are just examples to make it clear. it's more complex!

To me they looked the same as well and I had to distinguish them by their slightly different hair cut.

Actually, I thought he had 2 sisters and was living with one. I thought the 'girlfriend' mentioned something about 'their' sister, so I thought they were all related up until the sex scene, then I assumed I just misread the subtitle at some point, although after watching Oldboy.........