Fujiko Etou is a central figure at Constant Magic Academy, where she serves as the head of the girls' dormitory and is the current leader of the Etou family. Her rise to this position was precipitated by a tragic event: her older brother, the previous family head, died after failing a mission. She keeps his reanimated, partially amnesiac head preserved in a fluid-filled jar, with whom she frequently consults. Fujiko is also the leader of an underground movement that practices black magic, and she is one of the few individuals who initially understands the nature of Akuto Sai's prophesied future as the Demon King.

In terms of personality, Fujiko is calculating, manipulative, and highly intelligent, using her charm and authority to command respect and obedience from other students. She is openly ambitious and unafraid to pursue her goals, which initially involve using Akuto's immense demonic power to further her own plans for world domination. Despite her seemingly sinister motivations, she is not purely evil; her actions are largely guided by a deep, genuine attraction to Akuto, which eventually develops into love and shapes her role in the story. Her early scheme to magically enslave his loyalty backfires in a way that becomes a source of humor, and she effectively becomes one of his key allies.

Fujiko is a prodigious talent, considered one of the best students in the academy with a vast knowledge of magic and alchemy. Her magical abilities are unique and multifaceted. Rather than relying on direct combat spells, she specializes in the creation of powerful drugs, potions, and chemical compounds, which she deploys like grenades in battle. She wields a whip proficiently as her primary weapon and is skilled in necromancy, soul manipulation, and controlling demonic beasts. She owns a crystal ball used for clairvoyance and can create illusions and a field of blackness to obscure an area. While she is not a frontline fighter herself, her strategic genius is formidable; she can hold off armies through sheer cunning and later leads battalions of students in large-scale conflicts. One of her most potent abilities is the control of a powerful three-headed demonic beast named Cerberus, which serves as her primary combat asset. She also possesses a hidden research laboratory where she conducts her experiments.

Fujiko's relationships are central to her role in the narrative. Her primary focus is Akuto Sai, whom she initially sees as a tool to become the Demon Queen but later grows to genuinely love and support, often acting as a devoted follower. Her interactions with the justice-minded Junko Hattori are complex, as the two find themselves on opposing sides but are eventually drawn together as allies surrounding Akuto. She also has a unique and rather morbid bond with the talking, preserved head of her deceased brother, who often provides counsel and expresses concerns about her more dangerous plans. Throughout the story, Fujiko begins as a major antagonist but undergoes a significant character development, transitioning into a protagonist and a trusted member of Akuto's inner circle. Her evolution sees her shift from a self-serving manipulator to a leader who fights alongside her peers against larger threats, and her intelligence proves crucial in uncovering the true nature of their world.