Kisaragi Ninomai is a science teacher and one of five instructors acting as mother figures to an orphaned protagonist. Her gray hair falls to neck-length, and she has distinctive yellow eyes. She stands 1.59 meters tall, with a birthday on February 2, reflecting her name's link to the archaic Japanese month "Kisaragi". Her blood type is AB.

Though initially seeming emotionless and introverted, she displays considerable warmth, kindness, and an affection for cute things like baby animals and pink pajamas. She genuinely cares for those around her, especially the protagonist, whom she protects diligently. Her alien origin underpins her advanced technological skills and frequent scientific experiments, often involving unconventional, version-numbered inventions tested on the protagonist, sometimes leading to comical or hazardous outcomes.

Her scientific approach manifests in extreme methods, such as strapping the protagonist to a table for painful ear-cleaning experiments or employing devices like a "Booby Trap" headlock technique. These actions stem from a protective instinct and a drive to apply logical solutions. A key part of her past involves her childhood friend Kanna; together they once pledged to conquer the world. After their paths diverged, Kanna returns, confused by Kisaragi's maternal role. Kanna develops romantic feelings for the protagonist and repeatedly attempts to kidnap him, prompting Kisaragi to actively thwart these efforts using her inventions and vigilance.

Her responsibilities include safeguarding the household and concealing the living arrangement between the teachers and student to prevent scandal. She occasionally performs unconventional acts, like conducting a "black magic show" with the protagonist as her assistant, blending scientific presentation with theatrical flair. Across series installments, including OVAs and television seasons, her role consistently balances a logical, science-driven worldview with underlying compassion and dedication to her unconventional family.