Kim Su-cheol is Sang-gu's friend and protégé. 10 years ago, Sang-gu helped him out when he was beaten up by street thugs, inspiring Su-cheol to become a fighter and eventually becoming a champion boxer himself. After several years, he told Sang-gu that he wanted to retire from boxing so he could start a new life running a supply shop with his father and sister.
He willingly participated in a rigged underground MMA fight against Sang-gu in order to get the money he needed. During the fight, after Su-cheol refused to concede, Sang-gu violently knocked him out, putting him into coma, and Sang-gu was sent to jail for it. Sang-gu visits a still-comatose Su-cheol in hospital after his release, but he dies soon after. Sang-gu later discovers Su-cheol was suffering from CTE, which was his real reason for retirement.
