Saki Mishima runs a flower shop attached to her home in Ichi no Kuni, the human world. She is Yusuke Ninomiya's adoptive sister, though the precise nature of their family ties—whether she is the daughter of Yusuke's adoptive guardian or serves as his guardian while preferring the sibling title—remains unclear. Her life becomes entangled with Yusuke, his friend Haru, and their childhood friend Kotona. When Kotona is attacked and Yusuke needs help reaching her, Saki drives him there. Later, targeted by the Black Banner Assassin, she successfully defends herself, using her car as a weapon and escaping by driving into a river. Following Galeroth's defeat and Yusuke's choice to stay in Ni no Kuni, Saki loses all memory of him.

Saki's soulmate in Ni no Kuni is Bertha Roam, captain of Evermore's knights. This connection ensures they share core similarities across both worlds, including preferences and personality traits, with their lives intrinsically linked. Bertha exhibits fierce loyalty and bravery, especially in protecting Princess Astrid. She fights Black Banner soldiers but is ultimately defeated by Haru and taken for execution. Bertha survives unharmed after Haru and Yusuke defeat Galeroth and attends Yusuke's crowning ceremony. Her primary role involves safeguarding the kingdom and its princess within the fantasy realm's political and military structures.