Erich von Rerugen advances from Lieutenant Colonel to Colonel within the Imperial Army's Strategic Headquarters, shaping operational plans and high-level decisions. His initial encounter with Tanya Degurechaff during her training and early deployments cements a lasting perception of her as a monstrous threat behind a childlike facade, born from witnessing her ruthless combat efficiency.

Throughout the war, their frequent interactions in operational and strategic settings fuel Rerugen's apprehension. He observes Tanya's battlefield successes escalating conflicts beyond intended parameters. During the Republic campaign, he confronts her after unauthorized pursuit of retreating forces, criticizing her disregard for chain-of-command protocols and overstepped authority, revealing his fears about her destabilizing influence on operations and geopolitics.

Assigned to the Eastern Front, Rerugen's coordination of strategic responses strains his health, manifesting as recurrent stomach pain worsening under stress. This peaks after Tanya's raid on Moscow humiliates the Rus Union by destroying ideological symbols—an act Rerugen recognizes will provoke severe retaliation and prolong the war, intensifying his view of her as an escalation catalyst.

Despite persistent distrust, a strategic discussion where Tanya accurately predicts the Empire's victories uniting enemies through shared hatred forces Rerugen into reluctant respect for her geopolitical insights. This marks a shift from pure criticism to acknowledging her analytical capabilities, though he continues to deem her methods dangerously reckless.

Positioned as a moderating voice, Rerugen understands the Empire's growing international isolation from battlefield triumphs and advocates measured approaches. Yet he remains unable to counter the military's deepening reliance on Tanya's effectiveness, creating unresolved tension between his institutional loyalty and strategic misgivings as she deploys in high-stakes operations.