bad boy

LOVERS (K-drama) - He's a gangster
CRUEL CITY (K-drama) - Gangster
CRUEL ROMANCE (C-drama) - Triad Leader
INNOCENT MAN (K-Drama) - Revenge bad boy
FIVE FINGERS (K drama) - revenge bad boy plot (brill cast and acting)
OUROBOROS (J-Drama) - revenge
OH MY GHOST - Not main leads though (well depending on how you look at it)
WHITE CHRISTMAS (K-drama)
EASY FORTUNE HAPPY LIFE (Tdrama)

Several there for starters - depending on what you call a bad boy, ive listed all that are like that

These dramas/movies may suit what you are looking for~

Our Times - (Taiwan movie) An ordinary girl and a bad boy team up to win their high school crushes hearts. Set in the past (90's), told by the girl in the future.

Yankee-kun to Megane-chan - (Japanese drama) A boy who used to be a good student becomes a delinquent

Devil Beside You - (Taiwan Drama) Misunderstood bad boy known as 'Devil'

I hope I have helped ^^
Enjoy!!

Thank you for the recs : ).