Miu Takanashi is the second eldest of the three Takanashi sisters, a 10-year-old elementary school fifth grader. She inherited blonde hair from her Russian mother, while her unnamed biological father abandoned her mother during pregnancy. After her stepmother Yuri and father Shingo disappeared in a plane crash, she moved in with her uncle Yuuta Segawa to stay with her sisters, consistently calling him "Oji-san."

Miu displays maturity beyond her years, embodying an "Ace" through intelligence, athleticism, and fashion sense. She expresses a preference for older, mature men and actively probes Yuuta about his romantic life. Popular with boys at school, this attention sparks jealousy among female classmates who insult her, though a loyal female friend shields her. Miu occasionally uses a "Boyfriend Bluff," recruiting Yuuta to pose as her boyfriend and deter admirers.

Her tendency to help others serves as a coping mechanism for unresolved personal trauma, reflecting a "Stepford Smiler" dynamic. This surfaces when classmates pity her parental loss, triggering depression that requires intervention from Yuuta's friend Kouichi Nimura to restore her emotional stability. Miu acts as a "Shipper on Deck" for her elder sister Sora's affection toward Yuuta, encouraging Sora to confess her feelings herself rather than intervening directly.

Her character development includes choosing to remain with Yuuta and her sisters when presented alternatives, reinforcing her commitment to the family. Upon entering junior high school in volume 17 of the light novels, she launches a singing idol career. Her relationship with Yuuta subtly evolves, shifting from viewing him solely as a guardian to developing deeper, unspoken feelings alongside her established admiration for mature men.