Three hundred years before the adventures of Fia Ruud, a young princess named Serafina lives a quiet, isolated existence in the forests of the kingdom of Náv. Born blind and the second daughter of the royal family, she was sent away from the court as a young child to protect her from the prejudice and bullying her condition would invite. Her only companions are the gentle forest spirits and her devoted cousin, Sirius Ulysses, a young but formidable knight captain who frequently visits her.

At only six years old, Serafina's peaceful life is upended when an incident in the forest causes a immense, mysterious power to awaken within her. In a moment of panic, she taps into the abilities of a legendary Great Saint, miraculously regaining her sight and demonstrating a level of power that surpasses all known limits. This event sets her on an unforeseen path, pulling her back into the royal court and the complicated world she was sheltered from.

The central conflict of the series stems from this awakening. While Serafina possesses the power of the world's mightiest saint, she is still a child, and the political and social dynamics of the kingdom are fraught with danger for anyone with such abilities. The narrative follows her return to the palace under the protection of Sirius, who recognizes her potential and the threat it could pose. Together, they must navigate courtly intrigue, old prejudices, and new dangers while Serafina learns to control her staggering powers.

Key narrative arcs involve Serafina's transition from a lonely, blind princess in the woods to a pivotal figure in the kingdom. A significant plot point is her meeting with Canopus, a member of a marginalized race, whom she immediately befriends, using her power and influence to challenge the deep-seated prejudice against him and his people. Her innate, innocent kindness also allows her to form bonds with legendary magical beasts, such as taming a griffon and a fenrir, creatures that become her loyal companions. Throughout these events, Sirius acts as her steadfast guardian, both in awe of her power and fiercely determined to keep her true identity a secret from those who would exploit it. The story of Zero chronicles the childhood of the girl who would become the legendary Great Saint, revealing the origins of her hidden strength and the bonds that would shape the future of her world.