The world may have lost its last ninja Friday.

Hiroo Onoda, the World War II Japanese intelligence officer who died Thursday at age 91, drew from ninja-like military training to survive nearly thirty years fighting a long terminated war, according to ninja historian John Man.

“Other people would dispute this, but I’d say he’s the last of the ninja,” Man told TIME last year. Man devoted a chapter of his book Ninja: 1,000 Years of the Shadow Warrior to Onoda.

The intelligence officer was a graduate of the elite Nakano Spy School, which Man described as something of a modern ninja training ground. The school taught officers stealth tactics and values like integrity and — above all — completing the mission, which aligned with the basic tenets of the centuries-old traditional ninja practice.

Man wrote:

“The man who claims to be the inheritor of the ninja tradition, Masaaki Hatsumi, lists eighteen fundamental areas of expertise, eleven of which were echoed in Nakano’s training: spiritual refinement, unarmed combat, swordsmanship, fire and explosives, disguise and impersonation, stealth and entering methods, strategy, espionage, escape and concealment, meteorology and geography … There was enough in common between traditional ninja training and Nakano’s to call these men modern ninjas. The two shared loyalty, secrecy, a sense of duty, a sense of integrity.”

Onoda was deployed toward the end of the war to an island in the Philippines under orders to sabotage harbor and airport installations ahead of an expected U.S. invasion. There he remained for 29 years, hiding from search teams, stealing food and sometimes clashing with local civilians he thought to be the enemy. He refused to believe the war had ended until the retired Major who had given him his initial orders returned in 1974 to relieve him of his duty.

“Ninjutsu seems to have found its final and most extraordinary expression” in Onoda, Man wrote. “In his commitment to survive, in the techniques he used to do so, and in his humanity, Onoda … is in many ways the real last of the ninjas.”