Well, to be honest, I was a bit confused reading your topic post 'cause you mentioned 2 actors that had epic MAIN roles as an example, so I didn't really know how to respond. xD
Don't get me wrong, I think this is a great topic, just... maybe you should pick some other actors? :)
For me, second male lead or supporting role actors that first come to my mind are Lee Joon Hyuk in City hunter and Jo Jung Suk in King2Hearts... :)
Oh, and Yoo Ah In in Sungkyunkwan scandal! He was awesome as Geol Oh. Song Joong Ki was awesome there, too, as Goo Yong ha!
From the ladies, it's Go Joon Hee (Minsoo in Can you hear my heart!! I loved her there) and Lee Yoon Ji in Dream high and King2hearts!! Also, Yoon Jin Yi in A gentleman's dignity - she was adorable as Mae Ah Ri and it's her first role!!