Myu Nagumo is a young dagon girl rescued from slave traders by Hajime Nagumo and Shea Haulia in Fuhren. This rescue forged a strong bond with Hajime, leading her to first call him "Onii-chan" and later "Papa," a familial tie formalized when Hajime married her mother, Remia, becoming her stepfather. She joined Hajime's extended family, including Yue, Shea, Tio Klarus, Kaori Shirasaki, Shizuku Yaegashi, Aiko Hatayama, and Liliana S. B. Heiligh.

Physically, Myu possesses long wavy emerald-green hair, sky-blue eyes, fan-like fin ears, and webbed hands and feet. She typically wears a white frilly dress with blue ribbons, orange sandals, and a silver star-shaped hairclip. On Earth, magic earrings disguise her non-human traits, rendering her hair blonde and her ears human-like.

Kind, obedient, and compassionate, Myu occasionally displays spoiled tendencies, particularly concerning attention from Hajime. Early trauma, including kidnapping, accelerated her maturity, shifting her from tantrums to valuing empathy. She possesses innate "supernatural charisma," attracting and befriending mythical creatures (UMAs) like kappas, who willingly assist her without coercion or magic, a consistent trait since infancy.

Her development reflects influences from Hajime's family. She adopted otaku interests, immersing herself in anime and video games like Monster Hunter, and militaristic behaviors such as issuing drill-sergeant-style commands. Past traumas desensitized her to common fears like ghosts but instilled a phobia of deserts. She demonstrates courage in confrontations, adhering to Hajime's philosophy of showing no mercy to enemies. However, her eagerness to help others sometimes leads to unintended conflicts due to overlooked consequences.

Myu's combat skills evolved significantly through training with Hajime and his companions. She excels in marksmanship using customized toy rifles and dual-sword techniques with kodachi swords named "Muraymasa" and "Kolatetsu." Her fighting style incorporates defensive and counter-based maneuvers inspired by video games. She also mastered "Gun Kata" firearms techniques and barrier magic. Key artifacts include revolvers ("Donna & Schlak"), a whip ("This Whip is a Weapon"), and a ring ("Treasure Trove") storing items in dimensional space. Her arsenal includes magic gems ("Yue-oneechan's Love") and a smartphone capable of emergency healing gates.

She commands the "Demon Rangers," seven golems possessed by the Seven Princes of Hell, which combine into a large humanoid mecha ("Overanger"). Piloted via mental imagery, this mecha features flight capabilities, multiple weapon systems, and a transformation mode ("Overrun Deus Ex Machina").

Myu's relationships center on her deep attachment to Hajime, whom she fiercely defends against criticism, reacting strongly when Kouki Amanogawa disparaged him. Her presence softens Hajime's ruthlessness, serving as an emotional anchor. She maintains affectionate bonds with her adoptive mothers, particularly relying on Tio for protection during perilous situations.

Her journey includes adapting to life on Earth, where she balances childlike curiosity with combat readiness, symbolizing a blend of innocence and resilience shaped by her extraordinary family dynamics.