Akio Fudo hails from a town beneath the Sky Library, raised there by her mother following her father's death from illness. As Spriggan guardians, they protected the Sky Library and its legendary weapons until the entity Abyss Trinity obliterated the town out of boredom. Akio's mother perished in the destruction, using her final power to save her daughter. This tragedy shattered Akio's faith, shaping her research Thema as "Fides" (faith) and forming her core backstory.

Despite her past, Akio maintains a cheerful and optimistic personality, displaying composure even under threat. She is generally easygoing and accepting, extending this openness even to former adversaries like Arata. She enjoys bantering and teasing friends, often acting as an older sister figure who supported Mira in embracing her powers and mentored Arata in magic. Beneath her lightheartedness lies intense loyalty and protectiveness, igniting into fury upon learning Abyss Trinity caused her mother's death and prompting her to suggest extreme measures to safeguard allies. She also exhibits unexpected innocence, becoming embarrassed when losing clothing despite not being inherently shy. Her love for combat earns her the label "battle maniac" from friends, a title she embraces due to its alignment with her research focus.

Physically, Akio is tall and slender with long dark hair, typically tied with a red ribbon (later styled in a ponytail). Her standard attire consists of a white crop-top shirt, a long dark skirt featuring slits, and slip-on shoes. Excessive magic use triggers a transformation into a childlike "Hungry Mode" (or "empty stomach mode"), where she appears in a nun-like habit with a coif and veil, reflecting her Spriggan heritage. This form reduces her magical capacity but concentrates her attack power.

Her primary magic is Mantra Enchant from the Gula Archive, inscribing characters onto her body to enhance physical abilities. The magic's potency depends on her emotional resolve, with weak determination risking self-damage. She typically applies characters to her legs for devastating kicks (e.g., "Demon Crushing Descent") or her forehead for defense. This magic also enables knowledge transfer to other users and possesses healing capabilities. As a battle magus, she maintains zero magic consumption until attacking or defending. Even without magic, she demonstrates high proficiency in martial arts, agility, and tactical calmness during combat.

In *Heavens Library & Crimson Lord*, Akio joins an investigative mission to ruins near her destroyed hometown and the Sky Library. An energy beam transports the group to the Sky Library, where she repeatedly subdues the traitorous Lugh using Selina's camera and Lieselotte's binding magic. She later confronts Abyss Trinity, the destroyer of her town, triggering a rage-driven attack.

Throughout the series, Akio serves as Grimoire Security's third seat and Mira's partner. She attends Royal Biblia Academy as part of the Trinity Seven, undertaking missions such as investigating a Gula Breakdown Phenomenon in America, where she helps neutralize an undead nun causing zombie transformations. Her character arc finds partial resolution when Arata defeats Abyss Trinity, allowing her to reunite with her mother's spirit, reconcile her past, and develop deeper feelings for Arata, whom she calls "beloved."

Akio holds a part-time job as a chef at the establishment where Mira works as a waitress. She uniquely lacks a personal grimoire among the Trinity Seven and employs the incantation "On Basaradan Kan," derived from Kuji-In esoteric practices.