Arata Sagimori is a second-year high school student at Achiga Girls' Academy in Nara Prefecture and a protagonist of the spin-off manga "Saki: Achiga-hen episode of side-A". She is a childhood friend of Kuro and Yuu Matsumi; her family owns a bowling alley in the area. As a kindergartener, she grew up playing mahjong with adults at the Matsumi inn but stopped playing after her first year of elementary school. She is an admirer of Harue Akado and thus lost interest in the game after Harue stopped playing following Achiga's loss in the national tournament. When Kuro informs her that Harue has begun playing mahjong again, Arata agrees to join Achiga's mahjong club.

She was assigned on the position of Achiga mahjong club president by Harue as most level-headed member of the team.