Urano Motosu, a Japanese college student and aspiring librarian, perishes crushed by books during an earthquake. Her dying wish to live surrounded by books is answered when she awakens in the feverish body of five-year-old Myne in a medieval world. Books here are rare, handwritten artifacts exclusive to elites. The original Myne was a sickly, bedridden child prone to jealousy and tantrums towards her healthier sister, Tuuli. Urano's memories and personality merge with the existing Myne, forming a combined identity.

Myne possesses midnight blue hair (near-black), golden eyes, and pale skin due to chronic illness. Stunted growth makes her appear younger. She often wears her hair in a half-bun secured with a wooden pin. A defining trait is the Devouring, a magical illness causing lethal fevers under stress or mana overload, resulting from her immense, naturally accumulating mana as a commoner with no outlet.

Myne lives with her commoner family: soldier father Gunther, dye worker mother Effa, and sister Tuuli. Determined to recreate books, she invents products like homemade shampoo, hairpins, and culinary recipes using past-life knowledge. She befriends Lutz, a neighbor aspiring to be a merchant. They collaborate on paper-making using techniques like washi. Her business acumen leads to partnerships with merchants Otto and Benno, though negotiations reveal her initial naivety. When the Devouring nearly kills her, Merchant Guildmaster Benno intervenes with a magic tool, revealing nobles can manage the condition.

Discovering scriptures during her baptism ceremony shifts Myne's path. To access books, she negotiates becoming an apprentice shrine maiden. The corrupt High Bishop attempts to force her into servitude as a gray robe, threatening her family. In retaliation, her uncontrolled mana nearly kills him. The High Priest intervenes, securing her position as a blue shrine maiden with protected family access and retainers Fran and Gil.

Adopted by the archduke late in Part 2 due to political threats, she becomes Rozemyne, an archduke candidate. This role demands education in nobility, magic, and politics under Ferdinand's mentorship. She learns mana compression, defensive spells like wind shields, and blessings—sometimes unintentionally. Her magic affinity spans all elements; she later acquires divine protections from over 40 gods. As Aub Alexandria, she governs with Ferdinand, prioritizing library construction, temple reforms, and mana restoration for the land. Her rule includes adopting Letizia and shielding regions from political fallout.

Personality traits include obsessive determination for books, resourcefulness in recreating lost inventions, and protective loyalty toward loved ones. She shows compassion to strangers, driven partly by guilt that neglecting suffering hinders her reading peace. However, she often overlooks others' circumstances, rushing plans without consultation—a trait shared with her father. Cultural habits from Urano's life, like bowing apologetically, confuse those around her. Her aversion to violence and blood contrasts with her capacity for rage when family or allies are threatened.

Key relationships define her journey: her bond with Tuuli embodies mutual affection; Lutz evolves from a skeptical ally to a lifelong business partner who accepts her reincarnation; Ferdinand becomes her sternest mentor and eventual political partner; Benno guides her commercial growth while tempering impulsivity. Conflicts with figures like the High Bishop or hostile nobles highlight her resilience and defiance.

Long-term achievements include founding printing industries, standardizing paper production, establishing children's bibles and picture books, and reforming temple education to teach literacy and math to orphans. As ruler, she envisions libraries accessible across class divides, reflecting her core ideal that books should unite rather than exclude.