Saki Mishima operates a flower shop in the world of Ichi no Kuni. She is the adoptive sister of Yusuke Ninomiya, who addresses her as Miss Saki; her family took Yusuke in, and they share a close, caring bond. The precise nature of their familial ties remains ambiguous—it is unclear if she is the daughter of Yusuke's adoptive guardian or acts as his guardian herself while preferring the title of sister.

Her soulmate is Bertha Roam, residing in the parallel realm of Ni no Kuni. Their connection manifests through shared preferences and experiences, linking their lives across the two worlds. In Evermore, Bertha Roam serves as captain of the knights, demonstrating bravery and a protective nature, particularly towards Astrid, and wields a sword in her duties.

During a critical conflict, the Black Banner Assassin targets Saki Mishima in Ichi no Kuni. Rescued by Yu and Haru, Saki courageously fights back, using her car as a weapon against the assassin—who had transformed into a giant spider—and drives into a river to escape. Simultaneously in Ni no Kuni, Bertha engages Black Banner soldiers in combat. Overpowered by Haru, she is taken captive and faces execution. Both survive: Bertha is present at Yu's crowning ceremony after Galeroth's defeat, while Saki loses all memories of her brother Yu following these events.