Grandis Granva is introduced as the leader of a trio of jewel thieves, often referred to as the Grandis Gang, who initially serve as antagonists. She is a young woman of Italian origin who grew up in a wealthy family. Her past holds the key to her motivations: as a child, she was courted by a handsome man who turned out to be a swindler, ruining her family financially and emotionally. This betrayal led to her being disowned by her relatives and left her with a deep obsession for valuable jewels, which explains her relentless pursuit of the Blue Water. Only her loyal companions, Sanson and Hanson, remained by her side after her fall from grace, and the three of them formed their gang to steal gems.

At the start of the story, Grandis appears as a glamorous but comedic and somewhat bumbling villain. She is determined to steal the Blue Water from the protagonist, Nadia, and her methods often involve elaborate schemes carried out with her henchmen using their unique tank-like vehicle. Her personality, however, is far more nuanced than a simple antagonist. She is a good-natured person at her core, possessing a strong sense of loyalty to her gang and a surprisingly gentle side that emerges more fully as the series progresses. Her character undergoes a significant transformation when she and her gang are rescued by the submarine Nautilus after a battle with the Neo-Atlantean forces.

This event marks a turning point in her role in the story. Aboard the Nautilus, Grandis and her gang shift from being pursuers to allies of the main characters. Her motivations change dramatically; she abandons her quest for the Blue Water, becoming instead captivated by the mysterious Captain Nemo, for whom she develops an intense but unrequited infatuation. This new motivation puts her in a humorous rivalry with the captain's first officer, Electra, as both compete for Nemo's attention. Beyond her romantic pursuit, Grandis also evolves into a protective and maternal figure, particularly towards the younger characters like Nadia and Marie. She offers them advice and support, using her more worldly and pragmatic experience to guide them. Her earlier life of wealth and her later life of crime have given her a set of practical skills, including cooking, which she happily employs.

Key relationships define her journey. Her bond with Sanson and Hanson is one of unwavering loyalty; they function as her faithful retainers, supporting her schemes without question and remaining devoted to her long after her wealth is gone. Her relationship with Nadia is particularly significant. What begins as a predator-prey dynamic slowly transforms into a complex friendship and then a quasi-familial bond, with Grandis frequently acting as an older sister or mother figure to the often-stubborn and emotionally guarded young girl. Her romantic pursuit of Captain Nemo, while mostly played for comedy, also demonstrates her capacity for deep feeling and places her in direct, often amusing, conflict with Electra.

Throughout the narrative, Grandis displays notable abilities. Her most prominent talent is her command of a unique mechanical vehicle initially called the Gratan, though she prefers to call it Catherine. This beetle-shaped, six-wheeled tank, designed and built by her companion Hanson, is a marvel of over-technology for its era, capable of transforming for land travel, flight, and combat. Grandis serves as the vehicle's commander and guiding force, directing its actions during their various exploits. Despite her initial portrayal as a villain, her character consistently demonstrates resilience, loyalty, and an underlying warmth, making her journey from a self-serving jewel thief to a trusted ally and friend one of the enduring elements of the story. In the end, she is still seeking romantic love, but is shown to be living a vibrant life, popular and surrounded by admirers.