Yoshi Saeki is the daughter of Japan's prime minister and the fifth former wife of Genichirou Miyanomori. She is mother to Hiroshi Miyanomori, the family's fifth son, and shares a close bond with him. Yoshi actively encourages Hiroshi's inventive pursuits despite their frequent lack of success. Her own happiness is deeply connected to her son's well-being, leading her to defend him against her father's disapproval.

She advocates for women's advancement in society and openly opposes her father's authority, reflecting her commitment to social change. Described as kind, Yoshi enjoys reading. Her inclusive nature is shown through her support for Haru Asagi, the protagonist working as a maid in the Miyanomori household.

Physically, Yoshi possesses shoulder-length black curly hair and black eyes. Her typical attire consists of a pearl necklace, a white turtleneck dress layered under a black long shirt, and black high heels.