Lutz is a boy with short golden-blond hair and jade green eyes, initially appearing scruffy in heavily patched clothing due to his impoverished upbringing in Ehrenfest's lower city. As the youngest of four sons in a carpenter family, his older brothers often took his food and belongings, leaving him undernourished and struggling to secure his place. Before Urano's memories awakened in Myne, he occasionally interacted with her during her illnesses, acting protectively like an older brother despite their similar ages.

His friendship with Myne deepens after she teaches his family to make parue cakes from pulp waste during winter, ensuring he eats properly for the first time and leading him to vow unwavering support for her endeavors. He becomes her primary health manager and walking companion, carefully monitoring her physical limitations during her efforts to gather forest materials for early bookmaking attempts. When Myne reveals her dream of creating books, Lutz aligns his aspiration to become a traveling merchant with her goals. He facilitates her meeting with Otto, though Benno later guides him toward city merchantry instead. During their apprenticeship test under Benno to create plant-paper prototypes, Lutz deduces Myne's otherworldly origins due to her uncharacteristic knowledge and behavior. After confronting her, he pragmatically accepts her truth, reasoning no impostor would willingly inhabit such a frail body, and chooses to safeguard her secret.

As an apprentice under Benno at the Gilberta Company, Lutz diligently studies commerce, literacy, arithmetic, and merchant etiquette, overcoming his non-merchant background. Benno recognizes his honesty and capability, considering him a potential successor. A significant conflict arises when Benno plans to establish a paper workshop in another city and requires Lutz to travel. Lutz's father, valuing craftsmanship over merchantry, refuses permission, prompting Lutz to run away. Myne intervenes, escalating the matter to Ferdinand. A mediated meeting resolves misunderstandings, and Lutz signs a leherl contract with Gilberta, formalizing his merchant path.

Lutz's skills expand as he joins Benno's newly formed Plantin Company. He travels to Illgner to establish paper production and research new paper types, demonstrating leadership and adaptability. Later, during a mission in Groschel, he assists in setting up printing and paper workshops. When water pollution hampers paper quality, he and Gil compile local stories into a book as a gift for Rozemyne.

When Rozemyne relocates to Alexandria, Lutz chooses to move with the Plantin Company despite Benno offering a delay for his coming-of-age ceremony, prioritizing supporting the company's setup. Post-relocation, he lives in the Plantin building, with Benno and Mark as temporary guardians until adulthood. During his coming-of-age celebration at Tuuli's home, Rozemyne reveals a secret teleporter enabling clandestine visits. At this event, Tuuli and Lutz disclose their concealed engagement. Concurrently, Rozemyne announces her engagement to Ferdinand, positioning Lutz as Ferdinand's future brother-in-law.

Lutz evolves from a marginalized youth into a shrewd merchant. His loyalty, patience, and tenacity remain central, particularly in managing Myne/Rozemyne's health and recklessness. His carpentry background aids in practical problem-solving for book production, earning him the title "Gutenberg of selling books" from Rozemyne. His relationships, especially with Myne and Tuuli, underscore his role as a steadfast ally committed to enabling Rozemyne's dream of a world filled with books.