Heureka, also known as Bonnie in certain regions, journeys across Kalos alongside her older brother Clemont, Ash, and Serena. She actively engages with encountered Pokémon during their travels. Her personality features a playful tendency to seek romantic partners for Clemont, often despite his vocal objections.

In *Pokémon the Movie: Volcanion and the Mechanical Marvel*, Heureka joins Ash, Serena, and Clemont on a mission assisting Volcanion and Magearna. She shows compassion towards Magearna, reflecting her general fondness for Pokémon. Her role involves supporting the group at Nebel Plateau, a sanctuary for abused Pokémon, and participating in the confrontation against Councillor Alva in the Azoth Kingdom. During the film's resolution, she humorously proposes to Flamel, Kimia's personal assistant, on Clemont's behalf, prompting Clemont to use his Aipom Arm invention to pull her away.

Beyond the film, Heureka cares for a Zygarde Core nicknamed "Squishy," protecting it throughout the XY series. Her interactions frequently highlight her nurturing nature, such as comforting distressed Pokémon or joining group activities like fashion outings with Serena. Her dynamic with Clemont involves frequent sibling-like banter, alongside consistent concern for his well-being. She views Serena as an older sister figure, reinforcing the group's familial bonds.

Heureka's development focuses on growing independence and empathy. While cheerful, experiences like safeguarding Squishy and assisting in crises such as the Azoth Kingdom conflict demonstrate her increasing responsibility and courage. Her actions consistently align with her core traits of optimism and protectiveness within both the film and the broader XY series narrative.