Dramas where the male lead has a secret identity?

Something like Maekjong from Hwarang who's the king but pretends he's not. Something important like that. No gender bender stuff . The most important thing is that he HIDES something (like being a king), not that he's actually the king but didn't know. And possibly that he doesn't hide it to be with the girl he loves, but for more serious like political reasons, like Hwarang?

Yong Pal the main lead is a doctor but he cannot reveal who he is because it would be illegal medical pratice.

Healer he is an "errand boy" who does anything excepting for killing, but not exactly friends with the police and the law.

City Hunter I think Lee Min Ho also has a secrety identity here, can't remember exactly. But it is most for revenge.

The Guardians the main lead also has a secret identity, he is not himself in the workplace (revenge plot). Not romance. 

I dont know if this is what you are looking for, just what may fit your description.

Definitely second Healer.

Also: Love in the Moonlight (crown prince), Love O2O (college idol doesn't reveal his true identity in online game), School 2017 (high school vigilante), the King's Woman (king), the Masked Lover (undercover cop), and You Who Came from the Stars (alien).

Definitely recommend Healer, as others have suggested.

Another similar one would be Cambrian Period, in which the main lead hides his identity for crime/political reasons.