Fukumoto is a member of the D Agency, an elite spy training organization within the Imperial Japanese Army. He stands as the tallest of the agents, with a height of one hundred seventy-eight centimeters, and he is characterized by his black hair and black eyes. When considering his personality, Fukumoto is described as having a somewhat airheaded and absent-minded disposition. He is generally taciturn and does not easily express his emotions, making him a reserved and somewhat enigmatic presence among the more colorful personalities in the agency. This quiet nature seems to create a particular affinity between him and another secretive agent, Odagiri, and the two are noted to get along well. Beyond his espionage work, he has an unexpected domestic side, as he enjoys cooking and is sometimes seen handling kitchen duties for the D Agency members.

Within the narrative, Fukumoto serves as the central figure of an episode focusing on intelligence operations in Shanghai, a major city in China. His mission during this assignment is a masterful example of long-term deception and psychological manipulation. His role is not to gather information directly but to act as a puppet master, pulling the strings from behind the scenes to resolve a sensitive internal military issue. To accomplish this, he adopts two separate and convincing false identities: one is a low-level journalist named Saltzuka Saku, who claims to have reformed from a past involvement with leftist publications; the other is Kusanagi Yukihito, an activist and former classmate of a target. He uses this intricate web of lies to lead a young and idealistic military police officer, Honma, onto a specific investigative path. Fukumoto’s true objective is to expose the corrupt actions of a captain within the military police without causing a scandal that would shame the institution itself. He ensures that the revelation of wrongdoing comes from within the police force, thereby preserving its public honor while removing the problematic officer. His performance is so seamless that he is never suspected, and the perspective character of the story is unaware of his true influence until the very end, demonstrating Fukumoto’s mastery of covert operation, where invisibility is the greatest asset.

Fukumoto’s motivations are in line with the core philosophy of the D Agency, which is to gather crucial intelligence to protect the Japanese state without being swayed by traditional military notions of personal glory or patriotic fervor. He is presented as a professional who seems to take a quiet, almost imperceptible satisfaction in the artistry of his own meticulous work. Regarding his development as a character, Fukumoto remains somewhat mysterious, as the series focuses on the missions themselves rather than the internal growth of its agents. However, his ability to flawlessly execute an immensely complex plan built on years of planted backstory shows a complete mastery of spycraft and deep undercover work. Notably, he also contributes his specialized skills in a different operation, where his specific assignment is to retrieve key information from within a secure Nazi training facility in Germany, indicating his agility in diverse and high-stakes environments. His notable abilities include his strategic mind, his patience in cultivating false personas over long periods, and his talent for manipulating events and people without ever taking center stage.