Fate first appears with Arf and Bardiche, materializing in the town in search of Jewel Seeds. It is soon apparent that even Fate does not know why she is looking for these specimens of Lost Logia (the series’ term for ancient magical artifacts), but she does not care, stating “I must get them, because my mother wants them.” Fate is not impressed by her young, naive rival, Nanoha, and tells her that her quest is worthless, but Nanoha’s steadfast dedication to reforming Fate gets to her heart, and she begins to wane. However, this does not impress Fate’s mother, Precia Testarossa, who tortures the girl just for not getting all the Jewel Seeds. Fate remains loyal despite Arf’s protests; she remembers a time when Precia was kind and stable, and thinks that the Jewel Seeds will cure her mother’s insanity.
Fate teleports to her home from the top of an office building with complicated coordinates and a bit of magic. She lives in a gloomy hideout in the Garden of Time with her mother and Arf, sharing a room with the latter. She appears in the city whenever she senses a Jewel Seed, as she has no school and nothing else to do. This is also the reason why she spends much of the series transformed.
Late in the series, Arf finally strikes out at Precia, and is thrown out as “useless.” Fate finally starts to question her mother’s motives, as Arf has always been there for her, but still refuses to join Nanoha and accepts her challenge to a winner-takes-all battle for the Jewel Seeds. When Fate loses, Precia appears and steals the Jewel Seeds. Fate tentatively joins up with the Time-Space Administration Bureau, and it is revealed that her memories of the past were copied, and that she is a clone of Precia’s biological daughter, Alicia Testarossa. Fate doesn’t take this news too well, but attempts to convince Precia to give up; the madwoman laughs in her face, states that the Jewel Seed collection was an attempt to reach the legendary Al Hazard and resurrect Alicia, and snaps that ever since Fate’s creation, Precia has despised her. Fate, still hurt, attempts to save Precia, but is rejected as Precia and the preserved body of Alicia fall into a dimensional rift. Fate accepts Nanoha as a partner and a friend, and though she is arrested for what she did for Precia, promises to come back as she will probably be acquitted of all charges.
In A’s, Fate and Arf join the Time-Space Administration Bureau and go to Earth to team up with Nanoha again. Their new mission is to prevent the Wolkenritter from stealing mages’ Linker Cores (power source) for their Book of Darkness. Nanoha introduces Fate to her friends as a transfer student, and Fate has to get used to civilian life, cell phones and having a new mother figure in Lindy Haraoun. Early on in the season, Bardiche is upgraded to be able to compete with the Wolkenritter’s weapons, and is renamed “Bardiche Assault.” Nanoha and Fate meet a sickly girl, Hayate, whom both they and the Wolkenritter wish to save. At this point, Suzuka and Alisa discover Nanoha’s true identity. Fate is captured by the Book of Darkness and thrown into a dream from which she will never want to wake; a happy life with older sister Alicia and a sane, kind Precia. However, Fate remembers that her friends need her, and says a sad goodbye to the life she always wanted in favour of the life she has. With the help of the Wolkenritter and Hayate herself, they cure both Hayate and the Book of Darkness, which transforms into Reinforce, Hayate’s partner and weapon.
In an epilogue taking place six years later, Fate is older, has been adopted by Lindy, and working for the TSAB along with Nanoha and Hayate. She has changed her name to Fate T. Harlaown, the T presumably an abbreviation for Testarossa.
In StrikerS, Fate is a 19-year-old commissioned officer of the TSAB. She lives on Earth in Uminari City with Lindy, who has retired from her position as Admiral, having had too much excitement and seeking a desk job. Fate goes to school with Nanoha, Hayate, Suzuka, and Alisa. Though early promotional material suggested that she would abandon her battle uniform for a TSAB suit, a recent update of the nanoha.com website showed her in her Barrier Jacket, which appears to have grown to continue to fit her. She also seems to have gained a white cape, according to two promotional pictures; what the cape is for is unknown. In the manga, both the TSAB uniform and the Barrier Jacket are used with the Barrier Jacket overlaying or replacing the uniform when activated for combat situations. She took care of Caro and Erio when they were younger.