Genya Tachibana serves as a driving narrative force and the primary interviewer in Millennium Actress, his life and career intertwined with the subject of his documentary. He began his professional life in a minor position at Ginei Studios, where a fateful earthquake led him to save a young actress named Chiyoko Fujiwara from falling debris. Shortly after this incident, Chiyoko left the studio, unintentionally leaving behind a treasured key. Tachibana took the key for safekeeping, holding onto it for decades with the hope of one day returning it to her.

Years later, having become a director for Lotus Studios, Tachibana finally gets his opportunity. He seeks out the now-reclusive, elderly Chiyoko to make a documentary about her life as an actress. His personality, while ambitious and seeking, is marked by a deep, lifelong respect and care for Chiyoko, a devotion that borders on fanaticism. He is a man driven by a personal quest that mirrors the central theme of the film: a relentless, heartfelt pursuit. His motivation is twofold—to create a definitive record of her life's work and to finally return the key that has symbolized her own lifelong search.

Within the story, Tachibana acts as the catalyst for Chiyoko's reminiscences. As she begins to tell her life story, his role transcends that of a passive interviewer. He, along with his cameraman, is astonishingly pulled into her memories, finding himself physically present within the events of her past and the scenes of her films. In these shared visions, he often appears as a rescuer or benefactor to Chiyoko's younger self, repeatedly casting himself into her narrative as a figure of support and protection. His presence provides a crucial link for the audience, serving as an awed, sometimes bewildered participant in the unfolding of Chiyoko's life, while also offering moments of emotional clarity and humor through his earnest devotion. A key relationship exists between Tachibana and the unknown "man of the key," the revolutionary artist whom Chiyoko spends her life chasing. Tachibana views this figure as a romantic rival, and his determination to return the key is tinged with his own unspoken feelings. Tachibana also carries the burden of a painful truth he never reveals to Chiyoko: he knows that the artist was captured and tortured to death not long after he fled, a fact he keeps from her to allow her to keep her dream alive.

Throughout the film, Tachibana undergoes a significant emotional development. He begins as a simple documentarian and fan but evolves into a guardian of Chiyoko's emotional world. His final act is not one of revelation but of preservation, choosing to let her continue her eternal search even in death rather than shatter the illusion that has sustained her. His notable ability is not physical but narrative and emotional. His unwavering loyalty and presence shape the documentary itself, allowing him to literally step into the intersection of Chiyoko's reality, memory, and cinematic fantasy, becoming an active participant in the very story he set out to record.