Erich von Rerugen hails from an alternate World War I-era as a high-ranking Imperial Army officer, serving as Lieutenant Colonel and later Brigadier General within the Imperial General Staff. His responsibilities involve operational planning, battlefield inspections, and personnel assessments, including awarding commendations and overseeing units such as the 203rd Aerial Mage Battalion. He exhibits a diligent, observant personality, known for thorough analysis during strategic conferences. This background shapes his perception of Tanya Degurechaff as a dangerous entity disguised as a child, due to her ruthless efficiency and manipulative behavior; despite this distrust, he consistently supports her tactical requests in military operations. In the crossover setting, he serves as a teacher maintaining discipline among interdimensional students. His appearance retains key traits: glasses, blue hair, and a military-style uniform reflecting his rank. His personality shifts toward a more subservient and occasionally meek demeanor, frequently becoming the target of comedic mishaps, like being coerced into holding a watermelon for a destructive talent act. He maintains persistent wariness toward young female students, particularly extending his original distrust of Tanya to others such as Beatrice and Megumin due to their potential for chaos. Despite this apprehension, he demonstrates dedication to his teaching role and kindness, such as assisting students like Kazuma and Aqua with detention issues. His interactions with Tanya perpetuate their dynamic of mutual misinterpretation. He closely monitors her actions to prevent trouble, convinced of her capacity for malice, while remaining unaware she considers him a trusted ally. This contrasts sharply with his view of Emilia, whom he perceives as an "angelic" figure representing idealized normalcy, leading him to support her efforts positively. His ability, termed "Commanding Persuasion," aids in managing unruly students, though its full scope remains undefined. Relationships with other characters, like Vanir, remain ambiguous but suggest potential tension due to emotional reactions. His military expertise translates into administrative competence within the crossover setting, emphasizing organizational skills over combat abilities. His development consistently highlights his role as a foil to Tanya, his apprehensive yet responsible nature, and his adaptation to unconventional challenges while retaining core traits.