Rikka Hishikawa, known as Cure Diamond, is a central figure in the Doki Doki! Pretty Cure franchise. She attends Oogai First Middle School as a second-year student and serves as student council secretary alongside her childhood friend, Mana Aida. Rikka has dark blue eyes and chest-length hair, typically worn loose with a small braid secured by a ribbon; she sometimes wears red glasses. Her standard outfit is a white long-sleeved blouse, blue bow tie, light blue pleated dress, black socks, and short brown boots, switching to a sailor-style dress in summer.

Academically gifted, Rikka ranks within the top ten nationally in mock exams and leads her class. Her intense study focus can be disrupted by new hobbies, occasionally impacting her grades. Generally calm and introverted, Rikka deploys blunt sternness when necessary, often to temper Mana's impulsiveness. Their long friendship involves mutual protection: Mana defended Rikka in childhood, while Rikka now frequently intervenes to prevent Mana's recklessness. This dynamic extends to their student council duties, where Rikka's reliability balances Mana's enthusiasm. She maintains a close bond with another childhood friend, Alice Yotsuba, and builds trust with teammate Makoto Kenzaki after initial jealousy over Makoto's closeness with Mana.

Rikka transforms into Cure Diamond after her canine fairy partner, Raquel, deems her compatible with Mana. Initially declining, she accepts when a Lovead stone resonates with her memories of meeting Mana, enabling her to activate the Lovely Commune device and recite "Pretty Cure, Love Link!" to transform. As Cure Diamond, her hair lightens to indigo, grows to knee-length in a thick ponytail secured by a blue diamond accessory, and she dons a light blue dress with winged sleeves, diamond earrings, gold bracelets, and white boots. Representing wisdom and ice, she declares "The light of wisdom! Cure Diamond!".

Her primary attack, Twinkle Diamond, activates through the Lovely Commune, unleashing diamond-shaped energy beams to freeze foes. Secondary techniques include Diamond Shower (summoning diamond projectiles via the Love Heart Arrow), Diamond Swirkle (using the Magical Lovely Pad), and Diamond Blizzard (an enhanced ice attack performed jointly with Raquel). She debuted by pressing a weakness button to rescue the petrified Cure Heart from a Jikochuu. Her combat prowess evolved in episode 26 while defending the amnesiac antagonist Ira, crystallizing her resolve to act without regret and unlocking heightened power.

In the film "Mana Kekkon!!? Mirai ni Tsunagu Kibō no Dress," Rikka confronts memory alteration by the antagonist Marsh. Trapped in distorted recollections of elementary school, she initially lacks awareness of the altered reality but later collaborates with Alice to resist Marsh's henchmen. Her determination to preserve bonds with Mana and her friends overcomes the memory suppression. During the climactic wedding sequence in Mana's future, she appears in a blue dress with a small jacket and ponytail, differing from early promotional materials.

Rikka's core motivation stems from her childhood aspiration, inspired by her mother's medical profession, to become a doctor. This goal surfaced when Cure Ace challenged her to identify her "true feelings," leading Rikka to reaffirm her commitment to healing others. Her protectiveness extends to former adversaries, demonstrated when she sheltered the reformed Ira, consistent with her desire to aid the vulnerable.