Oslo is Otto's older brother. Although Oslo believes that Otto has the ability to be a merchant, he also believes that he is someone destined to get in trouble wherever he goes.
Regin is Otto's younger brother. Otto and Regin are shown to trust each other deeply. However, Regin often worries about his brother's wellbeing, believing him to be naturally unlucky. Regin sees Otto as the person who inspired him to become a veterinarian.
Otto knows Verhoe since his childhood. Otto has strong affection for Verhoe, considering her a friend and a member of his family. Although she sees Otto as a troublesome person, Verhoe is completely loyal to him.
Despite seeing Subaru as a problematic person, Otto starts considering Subaru his friend during the events that take place at the Sanctuary. Otto is willing to help him out whenever possible.
Marone and Otto are friends. Otto is fond of Marone and sees her as an admirably strong person.
Even before becoming an official member of her camp, Otto identified with Emilia's circumstances and decided to support her for the Royal Selection. The two currently have a friendly relationship. Otto usually gives her advice, which Emilia appreciates. She thinks of Otto as a very dependable person.