It's the remake of the Japanese series ホーリーランド (Holyland) that was originally aired in 2005.
Story: Kang Yoo doesn't fit in at school or anywhere else, he's just different. The ordinary teenager starts learning the basics of boxing from a book. When he goes out one day and gets attacked by a group of thugs he defends himself with his self-taught skills. In the aftermath he becomes known as the 'Hunter of Thugs'. When he meets Sang-ho by chance, he suddenly finds a mentor and starts to push himself to the limit to find out who he really is. During that time, he meets a lot of people, one of them being Tae-sik. Starting off as enemies the two become friends in the end.
Cast: Shin Dong-ho of U-KISS took the role of Kang Yoo. I'm still impressed with his acting, he would make a good actor. It's still sad he left the group and the whole industry but I wish him all the best and at least there are ways to see him. I wish he'd return as an actor some day though but this is not the right place to talk about this. But Yeo Hoon-min also from U-KISS joined him in this drama as Tae-sik. Those two were almost flawless actors. Sure, their characters had some issues but that's what made them appear human and one shouldn't confuse the actor and the character.
Actor Seong Woong was Kang Yoo's menthor, Sang-ho.
My rating is 8/10, it could've been longer with more details given on why exactly Kang Yu wants to learn boxing (or I just don't remember the exact reason) but then again I think there would've been more about a love story if there are more episodes. So in the end I'm not like "Woah, best thing ever!" but it's still pretty good and worth watching.