Tohai: Ura Rate Mahjong Tohai Roku

Not a lot of people know that one of the most talented and ruthless Mahjong players in the city is a young high school boy. Kei is a teenager of few words, but his observation skills, unwavering will and experience in the game allows him to survive in a world full of powerful, rich and dangerous characters. Kei is currently hiding a girl named Amina who has entered the country illegally and protecting her from being deported. He spends his days sleeping through class and his nights playing high stakes Mahjong, waiting for when a worthy opponent arrives.

(Source: MangaHelpers)