Here is only Forever 21 that is located in 명동, Seoul in South Korea.
It seemed more like Forever 21 is designed for westerners rather than for Korean people since there is so much more variety of competitively priced clothes, meaning you can often find clothes priced similarly to Forever 21.
Even for me (living in Ottawa), I feel like there are not many good stores and I end up going to Forever 21 for the price but I didn’t step into that store when I was in Korea. lol
Anyway.. A lot of people don’t know where this store is originated from. I took the info from Wiki.
Please make note of who owns the company! (Korean Family)
Currently they are some of the richest people in the world! I am so proud of them :))
Company: Forever 21
Founded : 1984
Headquarters: Los Angeles, California
Number of locations: 480 (2011) 
Key people: Do Won Chang
Founder & CEO: Jin Sook Chang
Chief Merchandising Officer: Linda Chang
Chief Marketing Officer: Esther Chang
Chief Visual Officer
Products: Clothing, Accessories
Revenue: Increase US$2.6 billion (2011) 
Total assets: $1.40 billion (2011) 
Employees: 27,228 (2011)