Fanatio, also known as Fanatio Synthesis Two, is a central figure in the events of the Underworld, holding the distinguished rank of Vice Commander of the Order of Integrity Knights. Her background is one of dedicated service to the Axiom Church, having lived for over a century as a knight. She is a woman of striking appearance, possessing long, flowing black hair and dark reddish-brown eyes, yet for most of her tenure, she concealed her features beneath a full helmet of purple armour designed with avian motifs. This concealment was a deliberate act, as she presented herself as male to her peers and subordinates.

The core of Fanatio's personality is defined by a profound internal conflict regarding her gender. She adopted a masculine persona because she genuinely believed that revealing herself as a woman would cause other knights, and especially her enemies, to underestimate her or refuse to fight with full seriousness. This fear of being patronized in combat led her to develop a unique fighting style that relied on keeping opponents at a distance, avoiding the direct sword clashes that she felt would expose her secret. Her behaviour in battle is often aggressive and dominant, and she has a noted habit of prattling on when she has cornered an adversary, revealing a more talkative side beneath her stoic exterior. Despite her efforts to hide her femininity, she secretly takes great care with her appearance, grooming her hair and applying light makeup, a practice she keeps hidden under her helmet.

Fanatio's primary motivation stems from her unwavering sense of duty as an Integrity Knight to protect the human empire and uphold the laws of the Axiom Church. However, this is intertwined with a deep-seated personal desire for respect and recognition as a warrior, not as a woman. Her greatest fear is being trivialized in battle, a fear that fuels her relentless pursuit of strength and her reliance on overwhelming long-range power. She also harbours a long-standing, unrequited love for her superior, the Knight Commander Bercouli Synthesis One. For over a century, she has watched her feelings go unreturned, a source of private anguish that is exacerbated by the presence of the younger and more beautiful knight, Alice Synthesis Thirty, whom Fanatio views with a mix of professional jealousy and personal resentment.

In the story, Fanatio serves as a major obstacle during Kirito and Eugeo's ascent of the Central Cathedral. She confronts them on the fiftieth floor alongside her four personally trained subordinates, the Four Oscillation Blades. During this fierce battle, Kirito manages to shatter her helmet, exposing her true gender to everyone present. Her worst fear seemingly realized, she flies into a rage and unleashes the full power of her divine weapon, the Heaven Piercing Sword. This weapon, forged from a thousand silver mirrors, possesses the devastating ability to fire powerful beams of light, a power that ultimately contributes to her own defeat. Mortally wounded by her own technique and the combined efforts of Kirito and Eugeo, she is saved only by the intervention of Cardinal, summoned by Kirito. This defeat becomes a turning point, as Kirito fought her without any regard for her gender, treating her as a purely formidable opponent, which begins to challenge her long-held insecurities.

Her key relationships are central to her character arc. Her devotion to Bercouli Synthesis One is a defining trait, a one-hundred-year love that she finally has the courage to act upon after the fall of Administrator. She also shares a significant bond with her subordinates, particularly the Four Oscillation Blades. Despite their lower rank and power, she cares for them deeply and trains them to fight as a unit to ensure their survival. The relationship with Kirito is catalytic; by defeating her and refusing to hold back, he forces her to confront the lie that her skill is dependent on hiding her identity.

Fanatio undergoes significant development from the War of Underworld onward. Having overcome her complex about her gender, she removes her helmet and stops concealing her femininity. Before the great war against the Dark Territory, she confesses her feelings to Bercouli, and they become partners. During the war, she serves as a key commander of the human defense forces, and after Bercouli's death in battle, she carries on his legacy, giving birth to their son, Belche. In the aftermath of the conflict, she succeeds Bercouli as the new Knight Commander of the Integrity Knights and becomes a leading member of the Human Empire Unification Council, governing the realm in the following years.

As a combatant, Fanatio is one of the most skilled swordswomen in the human empire. Her notable abilities revolve around her unique martial philosophy. Unlike knights who focus on a single, gallant strike, Fanatio is a master of consecutive hit techniques, employing rapid sequences of attacks. Her signature power is derived from her神器, the Heaven Piercing Sword. Through the Armament Full Control Art, she can unleash its memory as a high-temperature beam of light capable of piercing through almost any defense. In its most powerful release, the memory解放, the sword radiates beams in all directions, creating an area-of-effect attack that is as dangerous to the user as it is to the enemy, a testament to her willingness to risk everything for victory.