Eureka, sometimes called Bonnie, journeys through the Kalos region alongside her older brother Clemont, Ash Ketchum, and Serena. Though under ten years old and ineligible to be an official Pokémon Trainer, she actively engages with the group, caring for Pokémon like Dedenne, whom she carries in her satchel with Clemont's assistance.

She displays a fiercely protective instinct toward Clemont, frequently attempting to find potential marriage partners for him among older women they meet. These recurring matchmaking efforts often embarrass Clemont, prompting him to physically remove her from the situation. Unlike similar characters, she doesn't specifically target professions like Nurse Joy or Officer Jenny. Her bond with Clemont involves deep mutual trust; she relies on him in stressful moments while also balancing that reliance with challenging his overprotectiveness, understanding it stems from his care.

Eureka shares a close, sisterly relationship with Serena, joining her in activities like baking Poké Puffs and receiving fashion advice. She exhibits enthusiastic curiosity about new Pokémon, expressing excitement upon encountering them despite being unable to capture them herself. She occasionally refers to herself in the third person, a trait maintained in localized versions.

Accompanying the group to the Azoth Kingdom in *Pokémon the Movie: Volcanion and the Mechanical Marvel*, Eureka interacts with the mechanical Pokémon Magearna. Later, she proposes to Princess Kimia's assistant, Flamel, on Clemont's behalf, resulting in Clemont using his Aipom Arm invention to pull her away, continuing her established pattern. Her role in the film aligns with her series portrayal, emphasizing her familial loyalty and playful initiative.