Lot’s of scrolling 
Clue 1: royal couple has 2 babies (twins?)
Clue 2: twins not accepted, only 1
Clue 3a: baby 1 is a royal
Clue 3b: baby 2 has no money / is poor
Clue 4: princess dead & poor girl becomes princess
Clue 5: person with crown who isn’t princess (or queen?) is angry
Clue 6: Not North or South Korea, (go back in time) historical Korea
I have an idea, but let’s keep going… I’ll skip @my_happy_place 's clue till the end
Clue 3c: baby grown up as a child speaks Korean, Chinese and maybe more?
Clues 6a to 6d are really cementing the idea I have 
Clue 6a: supervillain woman with crown, not queen, is married to older king, king dies
Clue 6b supervillain couples with king, but he doesn’t want to marry her, so king gets crown taken away to supervillain’s content
Clue 6c: Soldier (higher rank than soldier?) is in love with supervillain
Clue 6d: supervillain is the nr 1 and teacher of the Hwarang
I’ll blur everything in case @lutra also wants to try.
Amusing, I might not have been able to guess if I didn’t watch it 
My answer: 
Deokman aka Queen Seondeok
I just read @my_happy_place 's answer. I’m sure 