My favorite underrated actors are:
Shin Sung Rok - He's mostly played villains or smls, but he truly is an incredible actor and I hope he gets more main roles. Fell in love with him when I watched Kairos.
Do Sang Woo - Hasn't had any main roles yet, only smls or supporting roles. But he is really good. I really like him in the currently airing drama The One and Only. Honestly his parts are the ones that shine the most in my eyes.
Not really sure if this next one is underrated because she has played some pretty major roles. But I very rarely see her talked about. And it's:
Lee Bo Young - I just love her soo much. Especially loved her in the drama Mother.