Big Bert Armstrong, known in the original Japanese version as Daijirou Goh, is the second eldest son of Ned and Mary Ann Armstrong and the brother of Steve and Little Jon Armstrong. His father, a hornless Boazanian noble who fled the oppressive caste system of his home planet to Earth, disappeared under mysterious circumstances when Big Bert was young. This event profoundly changed the boy, who had once been known for his playful and mischievous nature. Alarmed by his sudden shift in personality, his mother sent him to the countryside to train under a martial arts master. Through rigorous training and meditation, he became a master of several hand-to-hand combat styles and developed a particular expertise with the naginata and bo staff.

Big Bert's personality is defined by a quiet stoicism born from loss. He is the most reserved and serious of the Armstrong brothers, carrying the grief of his father's absence and, later, the death of his mother, who sacrificed herself to protect the Voltes team. Despite this heavy exterior, he possesses a deep well of compassion. He is fiercely protective of his younger brother, Little Jon, and has shown great empathy towards his teammate, Jamie Robinson, comforting her after her father's death by revealing a difficult truth about the man's illness. His motivations are rooted in family loyalty, a desire to defend Earth, and a personal struggle to come to terms with his complicated Boazanian heritage.

Within the Voltes team, Big Bert serves as the defense tactician and pilots the Volt Panzer, the massive component that forms the chest and torso of the super robot Voltes V. His role in the robot emphasizes defense, leveraging his grounded temperament and strategic awareness to protect the combined machine and his teammates during battle. While he generally follows the leadership of his older brother Steve, his intense personal drive can sometimes lead to friction, particularly when his desire to uncover the truth about his father causes him to act recklessly or disregard orders. His physical prowess extends beyond piloting; his mastery of martial arts makes him a formidable fighter in person-to-person combat, a skill he has demonstrated in various confrontations, including a coerced sparring match arranged by an enemy commander.

Key relationships shape his personal journey. The betrayal by Judalah, a Boazanian spy sent to infiltrate Camp Big Falcon, was a significant emotional wound. He had saved her life and fallen in love with her, only to have his heart broken when her true mission was revealed. His development throughout the story sees him confront the shocking truth of his own half-Boazanian lineage and the revelation that the invading Prince Heinel, also known as Zardoz, is in fact his half-brother. These discoveries force him to reconcile his identity as an Earth defender with his alien roots. Ultimately, his journey leads him and his family to Boazan, where they participate in the planet's liberation and are finally reunited with their long-lost father.