Black Diamond is a commander of the terrorist organization Big Fire, hailing from the planet Gargoyle. His appearance and mannerisms are notably defined by a fascination with the motifs of playing card suits, which is reflected in his chosen alias.

In terms of his role within the story, Black Diamond is depicted as a field operative who is frequently tasked with carrying out the organization's schemes on Earth. He is shown to be cunning and deceptive, often relying on traps and disguises to achieve his goals. On one occasion, he posed as a one-eyed traitor to lure the young hero Daisaku and his ally Minami into a trap, revealing his true identity only when they were cornered. His standard method of operation involves working alongside other Big Fire commanders, such as Doctor Over, with whom he coordinates missions to kidnap key individuals or steal sensitive information. He is also responsible for unleashing powerful monsters, known as kaiju, as part of Big Fire's attacks, including Ligon and Unbalar.

Black Diamond is portrayed as a persistent antagonist, but his repeated failures to defeat Giant Robo and the Unicorn organization define his character arc. After a series of unsuccessful missions, his standing with the organization's leader, Emperor Guillotine, deteriorates significantly. His final appearance comes in the episode featuring the synthetic monster Unbalar. Following the monster's destruction by Giant Robo, Black Diamond is not seen again and is believed to have been executed by Emperor Guillotine as punishment for his continual defeats. This marks the end of his direct role as a threat.

Regarding his personal motivations, little is explored beyond his loyalty to Big Fire and his desire to serve Emperor Guillotine's cause of world domination. His personality is best illustrated through his deceptive methods and his commitment to his assigned duties, even as his plans consistently unravel. He does not undergo any significant personal development or redemption, remaining a villainous figure whose story concludes with his implied demise off-screen.

Notable abilities attributed to Black Diamond include his skill in infiltration and espionage, as demonstrated by his successful impersonation of a fugitive to deceive the protagonists. He also has the capacity to command and deploy powerful kaiju, using them as his primary weapons against Giant Robo and the forces of Unicorn. He is shown to be a capable pilot and operative, able to escape from raids and engage in the various high-stakes missions assigned by Emperor Guillotine.