From MangaHelpers:
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 allow 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.