Anna Hendrix is a twenty-four-year-old woman who first appears to the Ferrari sisters as a helpful ally. She introduces herself as a publishing editor and a self-described Galileo maniac, a person deeply and enthusiastically obsessed with the historical figure Galileo Galilei. Her knowledge of Galileo and his works is extensive, and she uses this expertise to assist the three sisters on their journey, even naming their goldfish-shaped airship the Galileo. Her initial personality is marked by a strong sense of curiosity, abundant energy, and a willingness to put herself in harm's way to help the sisters solve the mysteries they encounter. This creates the impression of a devoted and somewhat eccentric fan of history who has become a valuable companion.

However, this outward appearance of a selfless helper masks a more complex and conflicted inner self. Anna is not simply a Galileo enthusiast but is also a spy working for the main antagonist, Roberto Materazzi. Her true motivation is a romantic obsession with Roberto, as she is unilaterally in love with him and carries out his orders to gain his approval. This duplicity is revealed through her secret actions, such as searching the Ferrari sisters' ship when they are not aboard and sending covert reports to Roberto about their movements and discoveries. Her role in the story is therefore that of a character caught between two opposing loyalties: her mission to serve Roberto and the genuine bond she begins to form with the Ferrari sisters during their shared adventures.

As the story progresses, Anna undergoes significant development. The time she spends traveling with the Ferrari sisters begins to affect her, and her initial loyalty to Roberto is challenged by a growing desire to protect the sisters from him. This internal conflict reaches a breaking point when she attempts to save them, leading Roberto to imprison her. This act of defiance, motivated by her newly developed care for the sisters, marks her transition from an antagonist to a sympathetic figure. She is ultimately freed by one of the Ferrari sisters, solidifying her break from Roberto's control.

Beyond her deep knowledge of Galileo Galilei and her skill at deciphering historical clues, Anna does not possess any superhuman or combat-oriented abilities. Her primary contributions to the group are her intellectual skills and her role as a temporary mentor figure. Her key relationships are defined by this duality: a one-sided, obsessive affection for Roberto that drives her betrayal, and the honest, if initially deceptive, friendship she builds with the Ferrari sisters, which ultimately redeems her. Her personal journey is one of shifting allegiances, moving from a servant of the story's main villain to a genuine, if initially flawed, ally of its heroines.