Daisuke Umon is the Earth identity of Duke Fleed, the crown prince of the distant and now-destroyed Planet Fleed. After the Vegan Empire, led by King Vega, invaded his home world and turned it into a toxic wasteland, Duke managed to reclaim the giant guardian robot Grendizer and escape. His flight brought him to Earth, where he crashed and was found by the scientist Dr. Genzo Umon. To protect him, Dr. Umon took Duke in as his own son, giving him the name Daisuke Umon and hiding his true origins.

In his daily life on Earth, Daisuke presents himself as a calm, level-headed, and hardworking young man, often employed at the Makiba family ranch. This composed exterior, however, masks a heavy internal burden. As the sole survivor of his royal family and his planet, he carries immense guilt over having fled and over the deaths of his people. Despite his peaceful demeanor, he is a warrior by necessity, and while he is thoughtful and composed, his deep-seated trauma and sense of responsibility for his powerful robot can lead to intense moments of internal conflict and determination when he must fight.

Within the specific context of UFO Robo Grendizer tai Great Mazinger, Duke Fleed's primary role is as Earth's main defender against the Vegan Empire. A key aspect of the plot involves the Vegans capturing Koji Kabuto and using him to commandeer the Great Mazinger, which had been kept as a symbol of peace. This places Duke in direct opposition to a former ally's machine, forcing him to pilot Grendizer to stop the rampaging Great Mazinger and save Koji. His motivation remains consistent with his broader character: to protect his adopted home of Earth from the forces that destroyed his own planet, even if it means fighting a legendary Earth robot.

His key relationships are central to his character. His bond with his adoptive father, Dr. Genzo Umon, is one of deep respect and gratitude, as the scientist provided him with a new life and identity. He forms a close, initially difficult alliance with Koji Kabuto, the former pilot of Mazinger Z, whose initial naivety about aliens frustrates Duke but whose bravery and skill ultimately earn his trust and friendship. Daisuke also shares a deep connection with Hikaru Makiba, who works with him on the ranch and later becomes his confidant and the pilot of the Marine Spazer, with their friendship holding the potential for romance. A pivotal moment in his life is the reunion with his long-lost younger sister, Maria Grace Fleed, who initially mistakes him for an enemy; their shared royal necklaces reveal their true kinship, giving Duke back a piece of his lost family. His tragic engagement to Princess Rubina of Vega ends in heartbreak when she sacrifices herself to save him from an assassin, an event that hardens his resolve against her father, King Vega.

Throughout his journey, Duke Fleed experiences significant development. He begins as a haunted refugee, concealing his identity and his pain. As the series progresses, he learns to trust his Earth companions, moving from a solitary warrior to a leader who relies on his allies in the fight against the Vegan Empire. His willingness to accept help and open up about his past marks his growth from a guilt-ridden survivor to a protector committed to his new home.

As a native of Planet Fleed, Duke possesses a unique physiology that grants him superhuman abilities, including enhanced agility, formidable strength, and an accelerated healing rate. He can materialize a durable pilot suit at will and has demonstrated a form of X-ray vision known as the Duke Vision. His most notable ability is his exceptional skill as a pilot, capable of controlling the immensely powerful Grendizer and its support vehicles, the Spazer and the Dizer trike, with great proficiency.