Kei Arakawa is a second-year student at Sangamaki High School in North Osaka and a supporting character of the spin-off manga "Saki Achiga-hen episode of side-A". She is Sangamaki's strongest player, but the team loses its prefectural qualifying tournament to Senriyama High School. Kei ranked second overall in the high school national individual mahjong tournament in the previous year. She is considered to be a monster player as she could play against both Rinkai's Satoha and Teru Miyanaga herself.