Name: Fate Testarossa, Fate Testarossa Harlaown (End of A's, StrikerS)
Age: 9 (Nanoha, A's), 12 (End of The Movie 2nd A's), 15 (End of A's), 19 (StrikerS), 23 (ViVid), 25 (Force)
Eye color: burgundy
Hair color: blonde
Intelligent Device: Bardiche Assault
Rank: S+, (StrikerS) Captain, Lightning Unit, Lightning One

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.