Main •
Woo Do-hwan as Kim Gun-woo :A sincere and considerate boxing prospect and a former
Marine. •
Lee Sang-yi as Hong Woo-jin :Gun-woo's partner in action and a former Marine. •
Park Sung-woong as Kim Myeong-gil (season 1) :The CEO of Smile Capital, and a notorious illegal loan shark who reigns above the law. •
Huh Joon-ho as Choi Tae-ho (season 1) :Formerly a legend of the private loan industry, he provides loans with no interest to people who have no money for medical treatment. •
Rain as Im Baek-jeong (season 2) : A menacing boxer who fights for money and power above all else. He would stop at nothing to get Gun-woo into the illegal boxing league Iron Knuckle Fighting Championship (IKFC), which he set up.
Supporting •
Choi Si-won as Hong Min-beom :A third-generation
Chaebol, and the eldest son of the Chairman of I'll Corporation. He has relatives who hold very high positions within the government. He is Gang-yong's younger cousin. • Park Ye-ni as Kang Tae-young :The leader of the Cyber Crimes Investigation Division. •
Park Hoon as Moon Kwang-moo :A private loan shark who formerly worked under Tae-ho, and a former Marine. •
Yoon Yoo-sun as Yoon So-yeon :Gun-woo's mother. •
Tae Won-seok as Kang In-beom :Myeong-gil's underling whom he met in prison.
Season 1 •
Kim Sae-ron as Cha Hyun-joo :Tae-ho's successor and foster granddaughter. •
Lee Hae-yeong as Hwang Yang-jung :Tae-ho's right-hand man who runs a Japanese restaurant. • Min Kyung-jin as Oh In-mook :Tae-ho's driver who was originally the security guard for Tae-ho's building. • Cheon Dong-bin as Cheon Dong-woo • Ha Soo-ho as Im Jang-do :The strategy management director of Smile Capital, who has connections with the police. • Jo Wan-gi as Kim Jun-min :A sales manager of Smile Capital. • Bae Jae-gi as Yang Jae-myeong :Myeong-gil's underling who pretends to be a homeless man in order to steal identification cards for loan purposes. • Hong Jun-young as Captain Jung :A service representative of Smile Capital, and a former fighter. •
Park Min-jung as Madame Im :A business partner and Myeong-gil's lover. • Jeong Da-eun as Oh Da-min :The granddaughter of In-mook, and an archer.
Season 2 •
Hwang Chan-sung as Yun Tae-geom : A former 17th Special Forces sergeant specializing in explosives who was dishonorably discharged, currently working with Baek-jeong as an enforcer. •
Lee Si-eon as Lee Man-bae : A former
NIS black ops agent currently working with Baek-jeong, specializing in operation planning, bribery and threatening. His real name is Lee Dong-hyeon. • Lee Myeong-ro as Allen : An IT professional working with Baek-jeong, and the IKFC site operator. • Kwak Seung-hyun as LOL : The host of the live streaming of the IKFC. •
Kang Min-ah as Gi-na : A casino dealer who communicates with Man-bae to distribute drugs. • Cha Ji-hyeok as Lee Woo-jeong : A detective of the Serious Crime Investigation Division of the National Investigation Headquarters, and Gang-yong's junior. • Hwang Dae-ki as Sun-bo : Kwang-moo's assistant.
Special appearances Season 1 •
Lee Joong-ok as a drunkard •
Ryu Soo-young as Lee Doo-young :Tae-ho's left arm who is loyal to him. •
Lim Hwa-young as Min-ji :Doo-young's wife. •
Kim Min-jae as Kim Jae-min (ep. 8) :A corrupt officer of the Serious Crime Investigation Division Unit 1, and a junior of Gang-yong.
Season 2 •
Gong Myung as Dong-hyun (ep. 1) : Gun-woo's fan, and a taekwondo practitioner. •
Ryu Kyung-soo as Choi Kyung-won (ep. 2–3) : A detective of the Incheon Seo-gu Police Station. •
Ha Young as Nurse Yoon (ep. 3) • Choi Kwang-il as Director Kang (ep. 5) : The director of the NIS, and Min-beom's uncle. •
Ryu Soo-young as Lee Doo-young (ep. 6–7) •
Park Seo-joon as Choi Sin-hyeong / Premium (ep. 6–7) : A NIS black ops agent. •
Kim Jin-young as Han Seul-gi (ep. 6–7) : A newly-joined NIS black ops agent working with Premium. •
Kang Shin-il as Heo Chung-ho (ep. 7) : The chief of Gunsan Gymnasium, and the former boxing mentor of Baek-jeong. •
Jo Hyun-jae as Myung-hwan (ep. 7) : Gang-yong and Woo-jeong's senior. • Park Sung-geun as Jeong Hee-jae (ep. 7) : The director of the Investigation Bureau of the National Investigation Headquarters. •
Lee Seol as An Ye-seo (ep. 7) : A NIS agent specializing in make-up and disguise. •
Lee Hyun-wook as Kang Hae-gwan (ep. 7) : A staff sergeant of the 17th Special Forces. ==Production==