Miu Furinji descends from the powerful Furinji and Kuremisago martial arts clans, inheriting exceptional physical abilities from both lineages. Her mother, Shizuha Furinji, died protecting infant Miu during an attack by Senzui of the Kuremisago Clan. Grandfather Hayato Furinji, the "Invincible Superman," discovered and raised her. Her father, Saiga Furinji, remained largely absent after joining the antagonistic organization Yami.

Extensive travel with Hayato during childhood involved rigorous martial arts training, which served as a coping mechanism. At age six, she encountered Kenichi Shirahama in his neighborhood, exchanging a yin-yang badge for his cat-shaped pin and effortlessly defeating local thugs. This encounter inspired both Kenichi and his friend Ryūto Asamiya to pursue strength, though Kenichi forgot it initially. Frequent moves prevented lasting friendships, leading to social isolation. At her previous school, she disguised herself with braided hair and non-corrective glasses to avoid jealousy over her appearance, athleticism, and academics, discarding this facade after Chikage Kushinada accused her of inauthenticity.

Miu maintains a cheerful, kind-hearted personality but exhibits intense concern for Ryōzanpaku's finances, reacting fiercely to wasteful spending. She manages household chores and budgets meticulously, once organizing children's martial arts classes to generate income post-dojō bankruptcy. Despite her gentle nature, she possesses a reflexive combat instinct causing her to throw anyone approaching from behind, a recurring comedic element. When close friends face grave danger, she enters uncontrollable berserker rages requiring physical intervention—such as Kenichi groping her breasts—to regain composure.

Romantic feelings for Kenichi develop gradually, though she remains initially oblivious to his affections. She exhibits jealousy towards other girls attracted to him, like Renka Ma, and becomes flustered when labeled a couple. After Kenichi saves her during a snowstorm-induced fever on a school trip—triggering repressed memories of her mother's death—she tearfully hugs him, showing increased physical affection. She later openly acknowledges her love, declaring faith in his ability to protect her and prioritizing his survival over her own. She ultimately marries Kenichi, adopting the surname Shirahama.

Her quest for familial truth drives significant development. Manipulated by Shō Kanō's offer of information about her father, she nearly joins Yami before Kenichi intervenes. Learning Saiga allegedly killed her mother, she attacks him with lethal intent, reconciling only after discovering Senzui was the true culprit and that Saiga always loved her.

Trained by Hayato since early childhood, Miu is a hybrid martial artist utilizing acrobatic kicks, aerial maneuvers, throws, and precision strikes. Her style incorporates Furinji techniques like the head-targeting "Benikawasemi" and her strongest attack, the drill-like "Kouhou Yoku." After temporary brainwashing by Silcardo Jenazard, she learns Pencak Silat fundamentals, including weapon forms and combos like "Juls Combo."

Her reflexes and sensory perception are heightened through "Rittoku No Kan," granting a sixth sense for danger. She masters defensive techniques like "Seikūken," creating an impervious barrier, and its advanced form "Ryūsei Seikūken," predicting and countering opponents through flow reading. Post-brainwashing training under Jenazard elevates her skills significantly, enabling her to outmatch adversaries like Rimi while holding back.

Key story involvements include introducing Kenichi to Ryōzanpaku, fighting Kisara Nanjō to protect him, and partnering with him in the D of D tournament. There, she demonstrates "Hyōi Kazekirihane," a bird-inspired special move, and battles Hayato in disguise. Confrontations with YOMI disciples highlight her growth, though masters like Raden Tidat Jihan overwhelm her. Her berserker episodes emerge when allies like Thor or Freya are gravely injured, often requiring Kenichi's intervention to prevent lethal outcomes.

Miu retains Kenichi's cat badge throughout her life, symbolizing their bond. She adores cats, often playing with them or gravitating toward cat-like individuals like Renka. Her hair accessories, gifts from her mother, conceal a blade and lock-pick, reflecting espionage skills acquired during global travels with Hayato.