Dokgo 3

Tae-Jin, who lost motivation in everything and drifted away, receives an offer from Shin Sang-Yun, the former captain of the wrestling club at Dangyeong high school, and eventually compromises with reality and enters the world of fists.
And as he adjusts to that world, he gradually begins to feel that he is falling into the abyss...
The saddest story of the Dokgo series unfolds.
(Source: KakaoPage, translated)