Otto serves as a soldier at Ehrenfest's southern city gate, working alongside Gunther. He has brown hair and brown eyes, presenting a young-looking appearance with gentle, calm features. His clothing suggests slightly greater wealth than his fellow soldiers. Originally a traveling merchant, Otto abandoned that path upon encountering Corinna, Benno the merchant's younger sister. Falling instantly in love, he spent his entire savings to purchase Ehrenfest citizenship and secure his soldier position, despite the risk of angering the merchant guildmaster who had arranged another marriage for Corinna. He considered this sacrifice necessary to marry her and moved into the Gilberta Company building where she resided, permanently forfeiting his ability to conduct official merchant business.

Otto retains merchant skills, demonstrating negotiation and bartering prowess. As one of the few literate soldiers at the gate, he handles nearly all paperwork, leading to frequent overwork during busy times. He resists training helpers, dismissing most candidates as unteachable "idiots." He teaches literacy and numeracy to new apprentices. Recognizing Myne's interest in writing, he provided her with a secondhand stone slate and taught her to write her name. Later, after she identified errors in his ledgers, he accepted her as an unofficial paperwork assistant. Payment was made in slate pencils, with Myne's official role listed as "student" to avoid child labor restrictions. Otto gave her advanced reading lessons after other students left, adapting his methods based on her feedback.

During an arranged meeting, Otto discouraged Lutz from becoming a traveling merchant, explaining it required surrendering valuable city citizenship and status, instead advising apprenticeship under a local merchant. He brought Benno to this meeting and expressed dismay when Benno proposed taking Myne as an apprentice, fearing the loss of her assistance. Otto demonstrated practical business acumen by advising Myne to seek investors and skilled craftsmen for product development. He traded nails for her shampoo recipe, settling for a demonstration for his wife when Myne declined to share the formula; Myne assisted Corinna with hair washing during this visit to their merchant district home.

Otto exhibits strong devotion to his family, frequently praising his wife Corinna and daughter Renate at length with minimal prompting, often irritating listeners. His name coincidentally shares pronunciation with the Japanese word for "husband" (夫). He notably spread rumors identifying Myne as Benno's "Goddess of Water." His background reveals initial plans to establish a store near Frenbeltag using citizenship discounts from his parents' local shop, abandoned for Corinna.