Hiroshi is a significant human character, introduced as the older son of the primary antagonist, Professor Gill. His primary importance to the plot stems from a unique physical attribute: the complete blueprints for a powerful superweapon, known as the Giant Devil, were tattooed onto the bodies of Gill's two sons. Hiroshi carries half of these complex plans, while his younger brother Akira bears the other half. This makes both brothers a constant target for the series’ villainous organizations, the Hakaider Squad and later SHADOW, who seek to capture them and claim the weapon designs for their own destructive purposes. Hiroshi is characterized primarily by his fierce protective instincts, particularly towards his younger brother. He fills the role of a protective older sibling, driven to keep Akira safe from the various forces that pursue them. Within the story, Hiroshi and his brother act as the central figures around whom much of the conflict revolves. Their guardian is a mysterious woman named Rieko, who has a connection to their father's past. Following her death, Hiroshi and Akira come under the care of Misao, who had previously been Hiroshi's tutor. While the series focuses on the android heroes, particularly Kikaider 01, who is awakened specifically to protect Akira, Hiroshi’s role is firmly established in the lore as a key to unlocking a terrible power, making his safety and the secret he carries a major driving force of the narrative. The available information does not provide details concerning a distinct character development arc or any notable personal abilities for Hiroshi beyond his role as the bearer of the weapon's blueprints.