Mia Koji, also known as Násti Yagyu, hails from a family with a Japanese father and French mother. Her grandfather, Professor Koji, taught romance literature at Sengoku University and held a profound belief in the legends of the Ronin Warriors and the Netherworld. Initially skeptical, Mia nevertheless assisted him in researching these tales. Her family possesses multiple properties, including two houses within driving distance of the city. Obtaining her driver's license at seventeen implies residence outside Japan during that period.

At the series' start, Mia and her grandfather witnessed disturbances heralding the Dynasty's return. After spotting Ryo Sanada on the news, Professor Koji identified him as a Ronin Warrior and tasked Mia with finding him. She returned with Ryo to discover her grandfather attacked and possessed by an evil spirit. Though freed, he succumbed to his injuries after revealing critical information about the armors' power source. His death deeply affected Mia, driving her to continue his mission and support the warriors.

Mia emerged as an indispensable non-combatant ally to the five Ronin Warriors. She offered them shelter first at her grandfather's residence and later provided lodging without charge. Serving as their emotional anchor, strategic advisor, and researcher into armor lore, her knowledge of the legends and ability to mediate conflicts proved vital against Talpa. Despite warrior concerns for her safety, she frequently accompanied them on expeditions, displaying a stubbornness that sometimes caused dangerous distractions. Her courage and focus persisted throughout her grief.

She formed a close bond with Yuli, acting as his primary caretaker and restraining his recklessness. Mia maintained significant interactions with Ryo Sanada, discussing tactics and mythology while supporting his well-being. Romantic implications varied across adaptations: the Japanese version hinted at deeper feelings for Sage, while the English dub suggested an inclination toward Ryo. Post-Message arc sources indicated a potential relationship with Sage, though unconfirmed.

Her involvement deepened in later arcs. During Gaiden, she traveled to New York with Yuli and Rowen to investigate Sage's disappearance. After uncovering his kidnapping, she and Yuli were abducted by Shikaiden's henchmen. Captive, she memorized a villain's computer keystrokes, enabling the warriors to disrupt control over Sage's armor. In Legend of the Inferno Armor, she aided Rowen in translating Mukala's incantation and journeyed to Africa, helping decipher cave inscriptions linked to the Inferno armors. Her persistence and observational skills remained crucial.

Mia's personality blended kindness, intelligence, and unwavering determination. She enjoyed cycling, driving, and romance literature but disliked secrets. Her loyalty and protective instincts align with Enneagram Type 6w5 ("The Loyalist"), emphasizing security and reliance on trusted authorities. She balanced the warriors' dynamics and humanized their battles, becoming an iconic support figure.