Myne, formerly Urano Motosu, was a Japanese college student and aspiring librarian whose intense book obsession defined her life. Crushed beneath her books during an earthquake at age 22, her final wish was for a life surrounded by books. She reawakened in the frail five-year-old body of Myne, a commoner girl in Ehrenfest. Myne was the second daughter of Gunther, a soldier, and Effa, a textile dyer, with an older sister named Tuuli. Before Urano's memories surfaced, the original Myne was sickly, weak, prone to tantrums, frequently bedridden by severe fevers, and resentful of Tuuli's health. A near-fatal fever shortly after her fifth birthday triggered the merging of Urano's memories and personality with Myne's, fundamentally altering her behavior and goals.

Physically, Myne possesses distinctive midnight blue hair, dark as the night sky, often pinned into a bun with a wooden hairpin, the rest falling loosely to her lower back. Her eyes are golden. Chronic illness gives her pale skin and a stunted frame, making her appear younger than her age. Homemade rinsham shampoo imparts noticeable glossiness to her hair. Her frailty severely limits physical activity, necessitating frequent rest and family care.

Myne's personality centers on an overwhelming fixation on books, driving her to recreate them in a world where they are scarce elite luxuries. She is resourceful, determined, and resilient, persisting through repeated failures in book-making. While self-identifying as selfish, she exhibits deep loyalty and protectiveness towards family and friends. She shows compassion to strangers in suffering, partly because ignoring their plight disrupts her reading peace. She harbors a strong aversion to violence, blood, and death, sometimes fainting at such sights. Impulsiveness and occasional lack of situational awareness cause conflicts, as she overlooks others' circumstances pursuing her goals. Cultural habits from her past life, like bowing to apologize, confuse those around her.

Her primary magical affliction is the "Devouring," a condition where her immense innate mana capacity—exceeding even the archduke of Ehrenfest by age seven—threatens her life without proper mana management tools or training. Unconscious emission of oppressive mana when enraged terrifies others and nearly caused fatal consequences, such as nearly crushing the High Bishop during a threat to her parents. Magically, she excels at blessings, capable of improvising prayers to multiple gods simultaneously and occasionally granting them unintentionally. Defensive wind-based spells, like Deflector Shields that detect evil, are among her earliest conscious magical abilities. Her mana eventually becomes a political and social asset, leading to her adoption into nobility.

Non-magical skills include advanced mathematical abilities, performing complex calculations mentally or with unique written methods uncommon in her world. She recreates past-life recipes, crafts, and technologies, such as pancakes from parue fruit pulp, decorative lacework, and hair ornaments. Her teaching skills are notable; she tutors soldiers, friends like Lutz, and temple orphans in reading, writing, and arithmetic. Musical training from her past life (three years of piano) combines with her new world's enhanced hearing to facilitate quick mastery of the harspiel under Rosina's instruction. Her needlework remains average despite introducing new techniques.

Myne invents numerous products using Urano's memories, including homemade shampoo, washi paper, ink, and printing methods. Her knowledge stems partly from her mother's DIY projects in her past life. Collaborating with craftsmen like Lutz and Johann, she refines these inventions, later honoring them with the title "Gutenberg" for their role in developing printing. Her efforts evolve into mass-producing books via printing presses, aiming to democratize literature access.

Relationships are central to her development. She idolizes her sister Tuuli, creating hair ornaments for her and sharing inventions. Lutz, her childhood friend and business partner, provides practical support. She earns loyalty from initially reluctant retainers like Gil and Fran through kindness and guidance. As the orphanage director in Ehrenfest's temple, she improves the orphans' lives by teaching them valuable skills, increasing their societal worth. Her bond with Hannelore of Dunkelfelger blossoms into a cherished friendship over shared book love. Noble figures like Ferdinand, the High Priest, become mentors, though their interactions are marked by clashes over her disregard for noble protocols.

Her character evolves significantly. To survive the Devouring and access noble libraries, she becomes an apprentice priestess, adopting the blue robes and eventually the identity of Rozemyne after her adoption by Archduke Sylvester. This integrates her into noble society, where she navigates political intrigue, including engagements canceled due to conflicts. She demonstrates fierce protectiveness, notably rescuing her adoptive sister Charlotte from kidnapping. As Rozemyne, she becomes a formidable mage, wielding offensive and defensive spells and participating in battles like the Battle of Gerlach, which leaves her traumatized until Ferdinand later restores her suppressed memories.

Her greatest achievement occurs near the story's conclusion, ascending as Aub Alexandria, co-ruling with Ferdinand. She establishes a library city; reforms temples; and restores mana-starved lands. As Mestionora's avatar, she shields regions from political fallout and advocates for equitable knowledge access, epitomized by reorganizing temple libraries using the Nippon Decimal Classification system. Her outbursts against library vandalism underscore her lifelong mission: creating a world where books are accessible to all, fulfilling Urano's dying wish.