Aguri Madoka, identified as Natalie Miller in the English adaptation, enters the narrative as a mysterious fourth-grade student. Her origins trace back to Trump Kingdom, where she previously existed as Cure Ace before arriving on Earth. Following a defeat against King Jikochuu, she reverted to infancy and was found and raised by her adoptive grandmother, Mari. Aguri’s true nature is revealed as the benevolent half of Princess Marie Ange's essence, dedicated to protection, while the malevolent half became Regina. This shared origin establishes her profound connection to Regina, whom she regards as a sister despite their opposing alignments.

In her civilian identity, Aguri presents as the team's shortest and youngest member, distinguished by burgundy eyes and long brown hair typically styled with pulled-back bangs. She favors a sleeveless red dress with white accents, magenta sneakers, and white socks, adopting warmer attire like a red jacket and burgundy boots in colder seasons. Her transformation into Cure Ace induces significant physical changes: she surpasses her fellow Cures in height, gains bright red elongated hair styled in distinct curls, and magenta eyes. Her costume features a red-and-black scalloped dress with a white petticoat, a tailored white jacket adorned with gold heart embellishments, feathered wrist pieces, and knee-length boots. A gold winged hairpiece and matching jewelry complete her regal appearance.

Aguri maintains a composed and wise demeanor beyond her years, often expressing herself elegantly with phrases like "Bravo" to appreciate food or "Adieu" when departing. Her strict adherence to rules can result in unintentional bluntness, balanced by a pronounced fondness for sweets such as peach rolls and anmitsu. Key relationships define her journey: she protects schoolmate Eru Morimoto from bullies, considering Eru her best friend; cares for the fairy Ai like a sister; navigates a complex rivalry-turned-alliance with Regina; and holds unwavering loyalty toward Mari. Her biological father, King Jikochuu, transitions from antagonist to reconciled family member after his restoration.

As Cure Ace, embodying the "trump card of love," she wields the Love Eyes Palette for transformation, requiring Ai to generate a Cure Lovead. Initially restricted to five-minute transformations due to energy constraints, this limit lifts after she gains knowledge from the Eternal Golden Crown. Her primary weapon, the Love Kiss Rouge lipstick, enables her signature Ace Shot attack, producing color-based effects: red for purifying enemies, purple for freezing, yellow for ensnaring with vines, and blue for trapping foes in bubbles. She also executes the Ace Mirror Flash using the Magical Lovely Pad. A red aura signifies her self-powered "Power Up" state during peak battle intensity. Notably, she and Regina temporarily assume the Cure Joker identities to confront threats to Trump Republic, strengthening their sisterly bond afterward.

Significant moments include intervening to save fellow Cures from a brainwashed Regina; withholding Mana’s Lovead to teach resilience; participating in Trump Kingdom's restoration after defeating Bel; and, in the epilogue, aiding classmates and preventing a satellite crash. Her name carries layered meaning—"Aguri" historically implied parents’ hope for no more daughters, while "Madoka" can mean "circle" or "yen." The kanji for Madoka ("en") combined with "Aguri" forms an anagram for "Regina," subtly foreshadowing their connection.