Millia, alias Hatsuki (Hair Oni), is a Middle Oni forged by the Demon of Divine Law Nobrega. She wields her long blond hair as a weapon. Her appearance features a broken Oni horn on her forehead's left side, typically concealed beneath a Sister's habit—a disguise maintained to hide her demonic nature, especially after Sumeragi declared "she is Millia because she wears the habit," following further power decline.

Initially resentful over lacking a unique race designation, she adopted the self-styled title "Kami-Oni" (Hair Demon). Early mockery from fellow demons for unconventional thoughts earned her the derogatory nickname "Henki" (Weird Demon). This bullying culminated in her slaying a high-ranking demon ally, costing her half her horn and halving her power. Deep shame forced her into years of flight from demonkind.

Sumeragi granted her sanctuary at Bremen Peter Church. There, she gained literacy, mastered magic, acquired knowledge, and received the name "Millia," a dialectal form of "Maria." She developed expertise in support magic, notably reviving the sealed Tree Demon Kiki. Defying her demonic origin, she learned to manipulate angelic energy—a rare feat for an underworld being—under Sumeragi's tutelage. Millia holds profound gratitude for this refuge and education.

Her personality merges rebellion with a pronounced combat obsession; she openly declares herself a battle maniac. When questioned about fighting, she states simply: "because I want to fight." She selectively seeks worthy opponents from strong invading nations, refusing indiscriminate harm. Though her battle-focus can suggest airheadedness, she possesses deep knowledge and cunning strategy. Trauma from past rejections fuels her constant struggle to reconcile her demonic heritage with her human-world experiences, driving introspection about her existence.

Tragedy struck at the church when she killed an abusive benefactor defending a close human friend, a nun. This very friend later rejected Millia upon discovering her demonic identity, inflicting lasting emotional wounds.

Sumeragi later tasked her with awakening the entity Juki in a forest. Encountering Sally's party, Millia met Meki (Carrot), a hornless, powerless former demon. Deeming Meki's state miserable, Millia resolved to end her life. Juki appeared, dismissed Millia as unnecessary after Sally effortlessly tore apart her hair attack, and assaulted her. Sally intervened, protected Millia, and killed Juki, prompting Millia's flight into the forest.

Later, suffering amnesia and targeted by assassin Chūki, Millia regained her memories during the attempt. To survive, she tore out her remaining horn, further diminishing her power. She begged Kibitsu Mikoto for her life and joined his group. Despite reduced power, her immense physical strength enables effective competition in martial arts tournaments, where she fights as a monk, knocking out human opponents. She works menial jobs like dishwashing to fund her substantial appetite.

In Legedia, seeking to regain Sumeragi's favor, she sided with him against Crow. Sumeragi maintained emotional distance. Domenica, using mind-reading, revealed his complex affection for Millia intertwines with memories of another from his past. Millia addresses her creator Nobrega as "Uncle," reflecting their intricate dynamic.

Her abilities, though diminished by horn loss, remain significant. Even after breaking her first horn, she matched Frau in combat. A later battle with Sally inflicted a permanent, non-healing wound on her side. Her fighting style leverages both hair manipulation and physical prowess.

She now travels with Kibitsu Mikoto and Dog, navigating a perilous world while wrestling with her dual nature and quest for belonging. Her journey embodies the ongoing tension between her demonic heritage, spiritual training, battle instincts, and moments of vulnerability or wisdom.