Akagi

Tokyo, 1958. A man seriously gambles at a mahjong parlor. He's playing against members of the Ryuzaki yakuza clan. If he loses, he must pay his debt through his own life insurance – truly a game of life and death. With his own existence at stake, he can only play passively, and the game comes to a head. Suddenly, a mysterious boy appears. His name is Akagi, a thirteen-year-old kid. The man senses a strong aura around the youth and decides to have Akagi substitute for him in the next round (Crunchyroll)