Fumika Narutaki is a student in Class 3-A at Mahora Academy, a large and prestigious educational institution. She is the younger of the Narutaki twins, her older twin sister being Fuka Narutaki. Physically, Fumika and her sister are the shortest girls in their class and possess a youthful appearance, noted as being among the few students who have not yet gone through puberty. She has pink hair and is often distinguished from her sister by her blue or light violet eyes. Born on December 6, she has blood type A.

In terms of personality, Fumika is characterized as being nearly the complete opposite of her more mischievous and energetic sister. She possesses a cautious and reserved demeanor, often acting as the voice of reason who tries to dissuade Fuka from their various pranks, though she frequently ends up going along with them anyway. Her personal hobby is cleaning, which she enjoys, and she also has a fondness for sweet foods. Conversely, she dislikes hairy or long things, such as caterpillars. As part of her activities at Mahora Academy, Fumika is a member of the Strolling Club, giving her a thorough knowledge of the campus grounds, and also serves on the Beautification Committee, which aligns with her preference for cleanliness.

Fumika's role in the story is often tied to her twin sister. The two claim to be undergoing ninjutsu training from their roommate, the ninja Kaede Nagase, and are occasionally seen wearing ninja outfits. Though their claimed ninja skills are rudimentary, they put them to use in various schemes, such as spying on their teacher Negi Springfield and his group to learn about their plans to travel to England. They once attempted to win the right to accompany Negi on this trip by trying to steal Kaede's badge, but their plan failed spectacularly. Despite her more cautious nature, Fumika shares the general admiration for Negi that exists among her classmates.

One of the most notable aspects of Fumika's character is her unique Pactio, a magical contract, with Negi. During a contest in Kyoto, she and Fuka both kissed a copy of Negi at the same time. This resulted in a botched or mistake Pactio, creating a single shared card for the two of them rather than an individual one. The name of this shared Pactio is Geminae Simulatrices. The abilities granted by this contract vary across different adaptations of the story. In the original Negima! anime, it allows the twins to don ninja outfits and use a shadow doppelganger technique, creating copies of themselves. In the Negima!? adaptation, Fumika's Pactio takes the form of a Cosplay card that turns her into a Western-style detective, complete with a magnifying glass that can see into the future of an object. A different Armor variant of her Pactio exists as well, depicting her as a ninja with a scroll and color-coded features opposite to her sister's to differentiate them.

Over the course of the series, Fumika's development, while not always in the spotlight, leads to a significant future. In the epilogue, she and her sister participate in the final battle to save Negi's father and defeat the Mage of the Beginning. Their personal lives also see a fairy-tale conclusion. The two sisters met a pair of mysterious animals around Christmas, who were later revealed to be magical princes in disguise. After graduating from high school, Fumika married one of these princes. She and Fuka are noted as being the first of their classmates to marry and have children, with Fumika eventually having a daughter and living with her family in a scenic old capital near Ariadne, within the borders of the Hellas Empire.