SEOUL BUSTERS! the name speaks for itself. Busting criminals across seoul- YES. A cop drama, hilarious (infact so hilarious that i maybe typing this from coffin-cause i died laughing [professional lame]), romance is not among the main characters but it is there in thier individual life, mystery and thriller is main. These both have an amazing part in the plot that is "disguise secret mission" basically were they disguise themselves as someone else to hid their identity. Also there is a actor who is in a major role in both of these. (its a secret) Give it a go guys!

both have revenge plot which makes it kind of similar. in glass mask the FML is tourtured for the reason that her father was a criminal so she assumes another woman identity to take revenge

they both have dark concept around a school theme which makes both of them similar. in extracurricular ,Determined to escape a dead-end life, a gifted high school student turns to a life of crime to ensure he can pay for college. I basically liked this drama and so I wanted to see Shadow Beauty.

Both are revenge kdramas. The series follows Kang Yi-kyung, a woman whose father is a murderer. She must deal with tough times i.e. her boyfriend leaving her, her family which adopted her also left her and she was harshly treated because she was a murderer's daughter so she plots her revenge. To fulfill her revenge, she assumes another woman identity and comeback as Seo Jung-ha.