Sachiko Arii, a widow still grieving her husband's passing, wears a traditional black mourning kimono symbolizing her enduring loss. Her reluctance to live alone leads her to provide shelter to Chako Hasegawa, the magical bunnygirl Delmo, and the owl-like Ikebukurou, reflecting her gentle, loving nature and desire for companionship.

She actively participates in the East Oizumi Academy cosplay club, sometimes judging their competitions. A blue-haired woman leaves purple roses at her husband's grave, later appearing in the audience during a cosplay battle; this woman is revealed as a limousine driver for the rival Shin-Takarazuka Academy Cosplay Club, implying an unexplored connection to Sachiko's past or her husband's memory.

By housing the club members and participating in their activities, Sachiko offers supportive stability within their community. Her character background maintains consistency throughout the series and OVA release.