Rikka Hishikawa attends Oogai City's Private Oogai Middle School as the steadfast best friend of Mana Aida. Calm, collected, and highly intelligent, she excels academically and upholds a strong sense of responsibility. Her parents, both doctors, inspire her medical aspirations, leading her to carry medical textbooks.

Her life transforms when the fairy Raquel selects her to battle the Selfish Kingdom as Cure Diamond, representing the Diamond Suit and the element of intellect. Her transformation phrase is "Diamond! The intellectual light that flashes through the heavens! Cure Diamond!" Wielding the Lovely Commune, later upgraded to the Lovely Pad, she employs intellectual strategies and precise attacks manifesting crystalline structures and light-based powers.

Initially, Rikka's unwavering loyalty to Mana causes her to prioritize Mana's needs, sometimes neglecting her own. This leads to conflict when she feels Mana is overextending or excluding her. Through her journey, Rikka learns to voice her opinions and challenge Mana when necessary, developing independence while retaining her supportive core. She bonds with Cure Heart (Mana), Cure Rosetta (Alice), Cure Sword (Makoto), and Cure Ace (Aguri). Her initial friction with Makoto evolves into mutual respect, and she serves as the team's analytical strategist balancing their dynamics.

Rikka confronts feelings of inadequacy and the pressure of her family's legacy, realizing her strength as a Precure is another way to help others. By the series' end, she balances Precure duties, academics, and personal growth, resolving to become a doctor while remaining a powerful protector.

In "Eiga Precure All Stars: Haru no Carnival," Rikka participates as Cure Diamond in large-scale battles against the film's antagonists. She contributes her intellect and powers to the unified Precure alliance, collaborating effectively across generations while maintaining her established role and personality.