Kazuki Ferrari is the middle sister of the three protagonists in Galilei Donna. She is a seventeen-year-old high school student living in Tuscany, Italy, with her family. As a descendant of the astronomer Galileo Galilei, she shares a unique heritage with her older sister Hazuki and younger sister Hozuki, though she does not initially feel she lives up to that legacy.

In terms of personality, Kazuki presents a cool exterior and often tries not to show her emotions, earning her the description of being the calm and logical type within the trio. However, beneath this composed surface, she is actually the most sensitive and emotionally vulnerable member of the family. She is quite timid by nature and prone to worrying, especially when faced with the dangerous and uncertain circumstances the sisters find themselves in. This contrast between her reserved demeanor and her inner anxiety is a defining aspect of her character. She is also known for skipping classes at school, which highlights her tendency to withdraw from group activities and her general disengagement from a life that feels unfulfilling.

Kazuki is a capable martial artist with strong physical abilities, holding proficiency in Shorinji Kempo. This skill makes her an effective Action Girl, capable of defending herself and her sisters in physical confrontations. Beyond combat, she is also a very skilled cook, a talent she sometimes uses to gently tease her older sister Hazuki, who lacks ability in the kitchen.

Her primary motivation throughout the early part of the story is a strong desire to return to her normal, everyday life. When the sisters are attacked by the mysterious organization Adnimoon and subsequently framed as terrorists, she is the most reluctant to join the global hunt for Galileo's hidden treasure. She sees surrendering to the organization or giving them what they want as the fastest way to be left alone and allowed to go home. This desire for normalcy is also linked to a romantic subplot; she has a crush on an unnamed boy from her school and keeps his picture on her mobile phone, a connection she is hesitant to sever even when it poses a security risk. This unrequited love represents the simple life she longs to return to.

A significant part of Kazuki's role in the story involves her fraught relationship with her own sense of self-worth. She is intensely jealous of her youngest sister, Hozuki, who is a mechanical genius capable of building incredible machines, seemingly inheriting Galileo's scientific brilliance. Kazuki feels useless and inadequate in comparison, believing she is the only sister without a special talent to contribute to their lineage. This jealousy and feeling of being an "Ineffectual Loner" create internal conflict, as she struggles to find her place on the team. Her character development is centered on overcoming this insecurity. Over the course of their journey, she begins to accept Hozuki's unique gifts and comes to understand that her own value lies in her physical strength, her loyalty, and her emotional perspective, which differ from but are just as important as her sister's genius.

Her key relationships are defined by her position as the middle sister. She is often caught between the headstrong and brash Hazuki and the intellectually gifted Hozuki. While she sometimes makes fun of Hazuki's failings, such as her cooking, the bond between all three sisters is the core of the narrative. Their initial estrangement and differing personalities create tension, but their forced cooperation as international fugitives gradually strengthens their familial ties, with Kazuki learning to rely on her sisters as much as they rely on her martial prowess.