Frey is a character from the anime and light novel series Unbreakable Machine-Doll. She is an eighteen-year-old third-year student at the Walpurgis Royal Academy of Machinart, where puppeteers hone their skills in magical combat. As an artificial human known as a "Promised Child," she has distinctive pearl-colored hair and crimson eyes, typically wearing her long silver hair tied back with a ribbon. Her birthday is May 2nd.

Frey's background is marked by tragedy and artificial creation. She is an orphan who, along with her younger brother Loki, was taken in by the magic research organization Divine Works. Her heart was replaced with a magical device, a dangerous modification that allows her to convert her own life force into immense power at the risk of death. She was raised under the care of an adoptive father named Bronson and a foster mother named Yomi.

In terms of personality, Frey initially projects a somewhat reserved and shy demeanor, but this is complemented by a kind and gentle nature. She has a well-known absent-minded streak, described as a natural airhead, which leads to clumsy and easily readable attempts at dishonest actions. This combination of shyness and a sweet disposition makes her appear soft-spoken and caring, particularly toward her brother and her automatic puppets, which she treats as family. Despite her gentle exterior, she possesses a strong will and a deep reserve of courage, willing to sacrifice herself to protect others.

Frey's primary motivation throughout the story is to protect and remain close to her younger brother, Loki. Initially, this protective instinct was exploited by her handlers, who ordered her to eliminate the protagonist, Raishin Akabane. However, after her failed assassination attempt and subsequent rescue by Raishin, she becomes a loyal ally and develops a romantic affection for him. Her role in the narrative evolves from an early adversary to a trusted friend and crucial member of Raishin's support network, often providing assistance during dangerous conflicts.

Her key relationships heavily define her character arc. Her bond with her brother Loki is central; he is fiercely overprotective of her, while she is equally determined to stand by his side and prove she is not a burden. Her relationship with Raishin transforms significantly after he shows her compassion, leading her to become one of his staunchest supporters. Her connection to her automaton, Rabbi, is also vital, as she views the three-headed hound as a beloved family member rather than a mere tool.

Throughout the series, Frey undergoes notable development. Initially portrayed as a low-ranking puppeteer with little confidence in her own abilities, she grows through relentless effort. She learns to master her powers, moving from being someone who needed protection to a capable fighter who saves her companions in critical moments. She evolves from a shy, easily overlooked student into a determined and self-sacrificing individual, earning recognition for her growth as a mage.

Frey's notable abilities stem from her modified heart, which can be unleashed to generate vast amounts of magical energy at the cost of her own physical health. She is a skilled puppeteer who initially commanded only her primary automaton, Rabbi, a forbidden "Cannibal Candy" class puppet that manipulates sound pressure. As she develops, Frey becomes capable of simultaneously controlling a full pack of thirteen Garm-class dog-type automatons, a feat that upgraded her combat rank and changed her fighting style to overwhelm enemies with numbers. Her signature technique involves removing her heart's limiter to activate a "Magic Heart Release," granting her explosive power but risking death from excessive blood and muscle conversion.