Eureka is the younger sister of Citron, the Lumiose City Gym Leader, traveling with him, Satoshi, and Serena across Kalos. Though too young to be an official Pokémon Trainer, she actively joins the group's adventures. Citron captured a Dedenne for her, and she diligently cares for it; the Dedenne often rides in her bag or rests in her arms, reflecting her deep attachment.

She habitually scouts for potential romantic partners for Citron. Upon meeting girls or women around her brother's age, she actively engineers meetings, frequently prompting an embarrassed Citron to use his gear to physically remove her from the situation.

During the journey, Eureka forms a significant bond with a small, green Pokémon called Squishy. This connection proves crucial to the storyline involving Zygarde and Team Flare's activities in Kalos, marking a key aspect of her character.

Later, Eureka reunites with Satoshi and Go in Miare City during events surrounding Satoshi's World Coronation Series battle against Elite Four member Drasna. She supports Citron while present for Satoshi's training and the subsequent battles.