Director Yamamoto leads ARCAM Corporation's Japanese branch, overseeing operations to secure and contain dangerous ancient artifacts called OOPArts. His duties involve coordinating field agents and managing responses to global threats from rival factions seeking to weaponize these relics.

He discovered Yu Ominae as a former COSMOS child soldier and took him in as a ward. Recognizing Yu's potential, Yamamoto provided training in military tactics, guerrilla warfare, and operational protocols to prepare him as a Spriggan field agent. He subsequently became Yu's primary mission handler, retaining command authority over his deployments.

Yamamoto displays strategic decisiveness during critical incidents. When COSMOS forces raided ARCAM's Japanese headquarters, he engaged in direct combat to protect the facility and personnel. His leadership extended to the South Pole expedition for the Fire Snake artifact, where he initially collaborated with ARCAM president Henry Garnum under duress. After Garnum allied with the Trident Corporation and endangered global safety, Yamamoto orchestrated a counteroperation. He coordinated ARCAM personnel to turn against Garnum and Trident forces, neutralizing their Antarctic base and containing the Fire Snake's destructive potential.

He maintains professional relationships with ARCAM members, including researcher Rie Yamabishi, whom he recalled to Japan due to her historical connection with Yu Ominae. His surname reflects the meaning "mountain base."