Fukiko Ichinomiya, known as "Lady Miya," is the illegitimate daughter of Mr. Ichinomiya and his mistress, Ms. Asaka, a former maid in the household. Raised by Mrs. Ichinomiya as her own, Fukiko learned the truth about her parentage on her adoptive mother's deathbed. She then discovered her younger half-sister, Rei Asaka, born from the same affair but not adopted by the family.

Her childhood contained significant trauma. At age eleven, encountering Rei after Ms. Asaka's death, Fukiko tearfully confessed her humiliation at a birthday party where she received low-status seating, making Rei vow to keep it secret. One year later, aged twelve, Fukiko became intensely infatuated with Takehiko Henmi, her brother Takashi's university friend, during his visit to the Ichinomiya villa. He recited Shakespeare's Sonnet 18 to her and promised to attend her birthday party, where she planned a violin performance. His failure to appear devastated her, leading to a suicide attempt by drowning. She preserved her room unchanged from the day they met and never again played the violin publicly. That winter, still heartbroken, she attempted a double suicide with Rei at a snowy beach. Fukiko slit Rei's wrist, leaving her unconscious in the snow, but regretted it immediately and sought help. Rei survived. Fukiko gave her a bracelet to conceal the scar, symbolizing a promise to complete their suicide pact someday. Rei subsequently moved out, received an allowance from the Ichinomiyas, and Fukiko gifted her a doll ("cherie la poupee") to signify constant surveillance.

As president of the Seiran Academy Student Council and unquestioned leader of the elite Sorority, which requires wealth, beauty, and social status, Fukiko is nicknamed "Miya-sama" (Princess Miya). She forms part of the "Magnificent Three" alongside Rei Asaka and Kaoru Orihara. Fukiko cultivates an image of elegance and refinement, typically wearing long skirts, a white blouse with a red bow, and a red headband. Her public demeanor is dignified and controlling, yet privately she displays extreme emotional volatility, shifting between tenderness and chilling cruelty. She manipulates social dynamics, believing competition strengthens individuals, and instigates jealousy among students vying for Sorority membership.

Her relationship with Rei involves psychological and physical abuse, rooted in their shared secret of illegitimacy and Fukiko's internalized shame about her status. She subjects Rei to tests of loyalty, such as forcing her to wait for hours in the rain under an elm tree, resulting in Rei falling severely ill. Despite the torment, Fukiko occasionally shows twisted affection, comforting Rei after inflicting harm. She sponsors Nanako Misonoo's Sorority entry not from benevolence, but to sabotage Nanako's relationship with Takehiko, whom Fukiko still loves obsessively. Unaware that Takehiko is Nanako's half-brother, she fears romantic feelings between them and uses Nanako's Sorority status to isolate her.

The anime adaptation shows subtle nuances in her development. Flashbacks depict her childhood vulnerability. Her final scenes imply a degree of remorse and "nobility," particularly concerning her treatment of Rei and the fallout from Rei's death. However, her manipulative tendencies persist, such as snapping a white rose during emotional breakdowns to symbolize fractured purity or control. She remains defined by unresolved trauma, aristocratic entitlement, and the destructive consequences of her unhealed heartbreak.