Akagi

Synopsis

A Genius who fell into darkness. The God of the Underworld. Akagi runs from a game of chicken where he almost died, to a Mahjong parlour where a man loses all his money to the yakuza. Akagi, who is only thirteen, tells the loser that his game needs improvement (although Akagi himself has never played before). Akagi replaces him in the game, and so a legend is born. When the police come to check on the place to see if the kid who ran from the game of chicken is there, the yakuza deny that Akagi was involved. In the end, the policeman invites Akagi to a much more organized Mahjong game, and from there, the legend of the Genius who Fell into Darkness continues to grow.

Synopsis & details from MangaUpdates