Daisaku Kusama is the central protagonist of the classic anime OVA series, a young boy who carries the immense responsibility of controlling the most powerful weapon on Earth. His story is rooted in tragedy, as he is the son of Dr. Kusama, a brilliant scientist who was forced to work for the criminal organization Big Fire. Under duress, Dr. Kusama created the nuclear-powered giant robot known as GR-1. However, fearing that his creation would be used for evil, Dr. Kusama gave his son a special voice-activated wristwatch, the only device capable of controlling the robot. Soon after, Dr. Kusama was mortally wounded while trying to defect from Big Fire, an event Daisaku witnessed firsthand. This loss, coupled with his mother's death when he was very young, defines his past and fuels his present.

Personality-wise, Daisaku is a complex figure shaped by a childhood that was ended too soon. He carries the weight of his father's legacy and the desperate, unanswered question his dying mother left behind: "Can happiness be obtained without sacrifice? Can time pass without sorrow?" Despite this burden, he maintains an indomitable spirit and a sense of childish wonder and optimism, though he constantly strives to act maturely among the adult members of the International Police Organization's elite team, the Experts of Justice. He is plucky, courageous, and shows moments of being an annoying or precious brat, but his fundamental drive is to fulfill his father's last wishes, topple the organization that killed his parents, and protect the world.

Daisaku's primary motivation is the pursuit of his mother's question and the fight against Big Fire, the shadowy cabal responsible for his family's suffering. His role in the story is unique, as he serves as both the greatest weapon and the greatest weakness of the Experts of Justice. While he has no superhuman powers, he commands Giant Robo, making him indispensable in battle. However, his youth and his reliance on the robot make him a vulnerable point that enemies can exploit. His key relationships are defined by his bond with Giant Robo, whom he treats as a friend rather than a mere machine. The robot itself seems to have a degree of sentience, often acting protectively toward its young master. Within the Experts of Justice, he looks up to his adult comrades as guides, developing a particularly close, sisterly bond with Ginrei.

Throughout the series, Daisaku undergoes subtle but significant development. He grows from a boy who simply commands a robot into a determined hero who truly understands the burden he carries. In a moment of crisis, when his father appears to him in a vision encouraging him to fight on, Daisaku solidifies his resolve, physically connecting himself to Giant Robo to continue their fight against overwhelming odds. His development is one of maturing into the responsibility his father entrusted to him. His notable abilities are few in terms of physical combat; he lacks any real fighting skills of his own. His true power lies in his will, his courage, and his exclusive control over Giant Robo using his wristwatch, which allows him to direct the robot's immense arsenal of missiles and cannons from a distance or from atop its mighty frame.