Saki Mishima, affectionately known as "Miss Saki" by her adoptive brother Yusuke Ninomiya, originates from Ichi no Kuni and operates the flower shop "Blumenhandlung Saki". Her life is intrinsically linked to Bertha Roam of Ni no Kuni through a soul mate connection, manifesting in shared preferences and deeply intertwined experiences across both worlds.

Targeted by the Black Banner Assassin, Saki faces a lethal attack thwarted only by Yu and Haru's intervention. During this confrontation, she demonstrates resourcefulness by using her car to combat the assassin after their transformation into a giant spider, executing a daring escape by driving into a river. Following Galeroth's defeat and Yu's choice to remain in Ni no Kuni, Saki loses all memory of Yu as her brother.

The precise nature of her relationship with Yusuke carries ambiguity—it remains unclear whether she is the daughter of Yusuke's adoptive guardian, making them siblings, or if she serves as his guardian but prefers the sister designation. Physically, Saki is depicted with brown or auburn hair, often styled in a ponytail, and brown eyes.