Since you have characters, it's now time to pick a location. Since the majority of your characters are Korean, you must go for South Korea. Where else would you put them in? Your hometown where you know where everything is to make it easier to write?! HA! Silly writer, no one does that anymore. Make sure your readers know that the setting is definately South Korea. They most likely won't figure it out by using other cities they won't Google Search so use/:

• Seoul 
• Itaewon
• Busan
• or any other South Korean popular city in a Bing Search.

When it comes to describing Seoul, don't use pictures to help you. Just imagine a lot of tall buildings, a ton of cafes, a lot of cars and entertainment companies that all seem to cluster together in that one city so idols can walk the street like they're just aveage Joes with little to no awareness that they are indeed famous Asian celebrities within the media. 

Also make sure you tell your readers what the hell you're doing in the middle of a country you have no knowledge of. Either you're:

• going to an university which just so happens to be next to this cafe which just so happens to be next to this entertainment industry where surprisingly your bias works.
• working at a cafe that your bias and friends go to all the time and at night before closing time
• just a traveler wandering the streets of Seoul with little to no idea how public transportation works because you didn't research about South Korea back home before you hopped a plane.