Direttore Yamamoto leads the Japanese branch of ARCAM Corporation, dedicated to securing hazardous ancient relics called OOPArts. He discovered former COSMOS child soldier Yu Ominae, took him as a ward, and trained him in military tactics to become a Spriggan operative, serving as Yu’s primary handler. Overseeing ARCAM’s strategic operations in Japan, Yamamoto provided intelligence on the Noah’s Ark artifact and U.S. Machine Corps involvement after Yu was injured in a school bombing by a brainwashed student. Despite advising against Yu’s engagement due to injuries, he coordinated logistics when Yu ignored orders and traveled to Turkey. Yamamoto consistently shows concern for Yu’s welfare, worrying when Yu escaped a hospital to commandeer an ARCAM plane and later facilitating his equipment—including an Orichalcum-fiber muscle suit—for a defense operation. His role expanded during a South Pole mission with researcher Rie Yamabishi to investigate the Fire Snake entity. Upon uncovering ARCAM president Henry Garnum and Trident operative Larry Markson’s plans to weaponize the artifact, Yamamoto allied with Rie against them, aiding in defeating their Antarctic forces and containing the Fire Snake. He maintains operational awareness, recalling Rie to Japan due to her history with Yu. His surname combines the characters for "mountain" (山) and "base/origin" (本), meaning "mountain base."