Otto is a supporting character who plays a meaningful role in the commoner-focused part of the story. His background is unusual among the commoners of Ehrenfest, since he was originally a traveling merchant without citizenship. During his travels, he met Corinna, the younger sister of the merchant Benno, and fell in love with her at first sight. He spent his entire fortune to acquire citizenship in Ehrenfest so that he could marry her, and their romance became well known enough that traveling minstrels later recounted it as a famous story. After settling in the city, he joined the city guard and served under Gunther as a soldier at the city gate. In practice, however, his duties were mostly paperwork and arranging supplies, tasks in which his merchant connections and experience were far more useful than combat skills.

Otto is one of the few commoners at the gate who can read, write, and do arithmetic, abilities he brought with him from his trading career. He used these skills to teach the children of soldiers how to read and write, which is how he met Myne. Recognizing her quick mind, he gave her a slate and trained her as his assistant, and she eventually took over a large share of the gate's paperwork. He also continued supervising her writing and helped her refine her letters while she worked under him.

In personality, Otto is calm and outwardly amiable, with a gentle, refreshing manner that masks a pragmatic and calculating nature. He is a former traveling merchant who thinks carefully about self-interest and consequences, and he gives advice in a composed way that often includes practical calculation. He is known for being a devoted husband who dotes on Corinna and can go on at length when bragging about her. He is patient with Myne and indulgent of her unusual requests, yet he also grounds her in the realities of the world, offering guidance that balances sympathy with clear-eyed practicality.

His motivations are rooted in securing his family's well-being and fulfilling his duties loyally. He remains conscientious in his work under Gunther and supportive of his brother-in-law Benno, while also taking a genuine interest in helping Myne and her friend Lutz find viable paths forward. When Lutz sought his advice about becoming a traveling merchant, Otto explained the harsh realities of that life and instead recommended Benno as a better option, steering Lutz toward a merchant apprenticeship. Myne, for her part, comes to rely on him heavily, which creates tension with Gunther, who feels jealous of his daughter's admiration for Otto and treats Otto brusquely as a result.

His key relationships define much of his arc. With Corinna, he shares a deeply affectionate marriage, and their daughter Renate completes his family. His connection to Benno is significant both personally and professionally; when Corinna became pregnant, Otto left his post at the gate to devote himself to the Gilberta Company, where Benno was working. Later, when Benno left to establish his own independent business, the Plantin Company, Otto rose to become the head of the Gilberta Company, a promotion that reflects the trust placed in his abilities.

Over the course of the story, Otto develops from a gate soldier with a hidden merchant past into a respected merchant leader, while remaining a steady source of support for Myne and her circle. His notable abilities include literacy, numeracy, and skill with paperwork, along with the logistical knowledge, trade connections, and negotiating instincts gained from his years as a traveling merchant. He is also a capable teacher, able to explain reading, writing, and practical matters clearly to others, and his measured, calculating temperament makes him a reliable adviser in difficult situations.