Otto serves as a soldier at Ehrenfest's city gate, partnered with Gunther. His dark brown hair and calm brown eyes complement an appearance slightly wealthier than fellow soldiers, evident in his finer clothing. Originally a traveling merchant lacking citizenship, Otto intended to purchase discounted citizenship near Frenbeltag to open a store using his parents' connections. This plan changed instantly when he saw Corinna, Benno's younger sister, and fell in love. To prove his sincerity, he spent his entire savings on Ehrenfest citizenship and secured his soldier position through merchant contacts, accepting the risk of angering the guildmaster and exclusion from official merchant business. He deemed marrying Corinna worth these sacrifices, and their romance became a popular tale for traveling minstrels. Post-wedding, he moved into Corinna's residence on the Gilberta Company building's third floor, and they had a daughter named Renate.

He retains his merchant mindset, frequently bartering and demonstrating sharp negotiation skills. Generally easygoing and cheerful, Otto becomes effusive when talking about his family, lavishing praise on his wife and daughter to the point of annoying listeners. Despite his soldier role, he identifies primarily as a merchant and maintains a pragmatic approach to work and relationships. His gate duties focus on paperwork, supply trading, and instructing apprentices in basic literacy, writing, and arithmetic. As one of the few soldiers proficient in these areas, he often becomes overworked during busy times. He is selective about training helpers, refusing most by stating teaching "idiots" wastes time.

Otto first meets Myne when she visits the gate with her sister Tuuli. Noticing her interest in literacy, he gives her a secondhand stone slate and teaches her to write her name. He later discovers her exceptional aptitude when she corrects errors in his accounting ledgers without an abacus. Recognizing her potential, he hires her as an unofficial assistant—officially registering her as a student to bypass child labor laws—and pays her in slate pencils. He provides her advanced reading lessons after other students leave. When Myne inquires on Lutz's behalf about becoming a traveling merchant, Otto discourages it, stressing that surrendering city citizenship is foolish and advising apprenticeship under a merchant instead. He arranges a meeting between Lutz, Myne, and his brother-in-law Benno, leading Benno to agree to mentor them in exchange for a plant paper prototype. Around this time, Otto trades nails for Myne's shampoo recipe, resulting in her demonstrating its use to Corinna.

As Myne's influence grows and Benno's responsibilities increase, Otto and Corinna assist in managing the Gilberta Company. Near the time Benno founds the Plantin Company, Otto transitions into co-leading the Gilberta Company as its head. He also spreads the rumor identifying Myne as Benno's "Goddess of Water." His merchant background and familial connection to Benno solidify his ongoing role in Ehrenfest's commercial activities.