Sir Isaac Cavendish is a British intelligence operative operating undercover in the fictional European nation of Kuchen during the tense interwar period. His official cover presents him as a writer researching life at Kuchen's prestigious Rosenstolz Academy. His true mission, assigned by the British government, is to evaluate whether neutral Kuchen can be influenced or coerced into supporting British interests, potentially including the use of force. He works alongside other undercover agents within Kuchen's volatile political climate.

Born into nobility that succumbed to poverty, Isaac endured significant hardship in his youth. His family faced forcible eviction from their ancestral manor due to financial ruin, an event that deeply scarred his younger sister, Rosa. Left responsible for Rosa and his two younger brothers after becoming separated from his parents, Isaac joined the British Army to support them, serving through the "hell" of World War I. His exceptional combat conduct led to recruitment into the British secret services, with his dedicated service ultimately earning him the title of "Sir."

While in Kuchen, Isaac interacts extensively with Rosenstolz Academy's Strahl class students, particularly Orpherus, Eduard, and Ludwig. He initially expresses disappointment in their idealism and perceived detachment from global political realities, challenging Orpherus's views on governance and peace by arguing words alone cannot prevent looming war. Despite his mission's objectives, he exhibits moral complexity, refusing bribes from fellow agents and intervening to protect Orpherus and Ludwig during danger at a break-in investigation in the headmaster's office.

Isaac leverages connections with Rosenstolz alumni to gather intelligence. He is resourceful, utilizing espionage tools like a revolver and a motorcycle. His soldier's past surfaces occasionally, particularly when sharing stark wartime experiences with students to emphasize conflict's brutal realities.

Later developments reveal his involvement in critical events, including treating Orpherus after an assassination attempt and investigating plots against Kuchen's king. His actions suggest a growing tension between his duty to Britain and his personal investment in the students' fates and Kuchen itself. He also encounters figures from his past during operations, hinting at a broader personal history intersecting with his espionage.