Eureka, sometimes called Bonnie, travels as the younger sister of Citron alongside him, Satoshi, and Serena. Though too young to officially be a Pokémon Trainer, she deeply cherishes Pokémon, often carrying her brother's Dedenne in her satchel. She bubbles with excitement and curiosity encountering new Pokémon, instantly finding Diancie adorable.

During Diancie's quest for Xerneas, Eureka bonds with the Mythical Pokémon through shared experiences like shopping for clothes and trying on dresses, showcasing her youthful enthusiasm and introducing Diancie to human customs. Eureka consistently offers emotional support, reassuring Diancie of her inherent worth beyond creating diamonds, bolstering the Pokémon's confidence.

Her protective, nurturing nature is clear when she insists on keeping Diancie's temporary diamond as a memento, knowing it will vanish. Persistent efforts to find a bride for her brother, Citron, demonstrate her devotion, sometimes creating comedic moments. While not a fighter, Eureka contributes through optimism and empathy, comforting Diancie during doubt or fear.

Amid the Diamond Domain crisis involving Yveltal and Xerneas, Eureka's role centers on providing Diancie steadfast friendship and moral support.