Daijirou Gou, sometimes called Robert "Big Bert" Armstrong, serves as the second eldest Armstrong brother. His father, the hornless Boazanian noble Ned Armstrong (originally Hrothgar), escaped enslavement to Earth and married Mary Anne Collins Armstrong, rendering Daijirou half-Boazanian – a heritage initially unknown to him. Daijirou displayed a playful, rebellious spirit in childhood, but his father's unexplained disappearance profoundly altered his personality. Concerned by this change, his mother sent him to the countryside for training under a martial arts master. There, he mastered diverse hand-to-hand combat styles, meditation, and specialized in weapons like the naginata and bo staff, developing a distinct preference for long, stick-like armaments.

Following his father's departure and the subsequent Boazanian invasion of Earth, Daijirou joined the Voltes team at Camp Big Falcon alongside his brothers Steve and Little Jon Armstrong, plus allies Mark Gordon and Jamie Robinson. He pilots the Volt Panzer, forming the chest and torso of the combined robot Voltes V. His combat role emphasizes defensive tactics and close-quarters engagement, leveraging his martial prowess.

Daijirou harbors a profound attachment to his missing father. This intense focus sometimes drives recklessness, leading him to disregard mission protocols and Steve's leadership in attempts to locate or rescue Ned, creating friction. Their shared dedication to family ultimately reconciles them. Beyond combat, Daijirou exhibits deep empathy, particularly comforting Jamie Robinson after her father's death by revealing his own father's terminal illness to alleviate her guilt. He also demonstrates fierce protectiveness towards younger brother Little Jon, rescuing him from Boazanian threats.

Romantically, he develops feelings for Judalah, a Boazanian spy infiltrating Camp Big Falcon. Discovering her true identity and motive – coerced espionage due to her sister being held hostage – causes significant heartbreak. Judalah's subsequent failed assassination attempt on Prince Zardoz and escape further compounds his emotional turmoil. Physically, Daijirou is depicted as a large, heavyweight man with short dark hair, typically seen in a dark green pilot uniform or casual attire like an orange T-shirt and jeans. Despite rigorous training, he retains a heavy build. His character arc navigates loss, betrayal, and loyalty while maintaining his role as a steadfast Earth defender and supportive brother.