Himari Arisugawa, a second-year at Ichigozaka Middle School, possesses extensive knowledge of sweets and their scientific properties, earning her the nickname "Sweets Doctor." Her passion for sweets science frequently leads her into detailed explanations, punctuated by catchphrases like "Sweets Are Science!" and "Let Me Explain!" Despite this expertise, past experiences where her enthusiasm drove others away left her deeply shy. She struggles with social interactions, particularly around strangers or under scrutiny, making her hesitant to speak openly to those she doesn't know well. Physically, she has long golden-brown hair styled in low braided tails with short bangs and brown eyes. Her typical attire includes a white long-sleeved blouse under a yellow short-sleeved shirt adorned with a green ribbon and apple charms, paired with a mint ruffled skirt, teal socks, and brown boots.

Her journey as Cure Custard begins during a pudding-making attempt with friend Ichika Usami. A fairy named Pulupulu steals the "Kirakiraru" (positive energy within sweets) from their creation, causing Himari's squirrel-decorated pudding to transform into an Animal Sweet and a Sweets Pact, enabling her transformation. As Cure Custard, representing intelligence and courage with squirrel and pudding motifs, her appearance changes: orange hair in a curled side-tail, squirrel ears, and a large striped tail. Her costume features a yellow dress with white ruffles, a brown caramel-sauce-like peplum, a red cherry choker, white opera gloves with red cuffs, and orange squirrel-themed shoes. She introduces herself: "With wisdom and courage! Let's・La・Mix It All Up! Cure Custard! Ready to Serve!" Her abilities include squirrel-like super-speed and attacks using yellow energy blasts or sticky ropes from her Sweets Pact wand. She later gains the "Custard Illusion" defensive technique.

Key to her development is overcoming social anxiety. Tasked early on with promoting sweets in a video, she freezes from stage fright. A breakthrough occurs when a young girl questions the star-shape of churros. Recalling her own childhood fascination with custard pudding's roundness, which sparked her sweets science research, Himari explains the ridges' structural purpose (preventing explosions during frying). This experience reveals she can use her knowledge to connect with others. She subsequently gains confidence for public roles like reporting and participating in a televised sweets audition.

Her growth is bolstered by mentorship from Tachibana Yuu, author of her favorite book, *The Science of Sweets*. Impressing him with her knowledge, she assists his "Great Sweets Experiment" to uncover sponge cake secrets. Though the experiment fails initially, Yuu commends her dedication, reinforcing her self-worth. During this episode, she defiantly protects stolen Kirakiraru from an antagonist, showcasing increased resolve. Her Animal Sweet later evolves into a squirrel form, symbolizing her progress. Civilians, she becomes more open, forming a close bond with the contrasting Aoi Tategami.

In the series epilogue, Himari's adult future sees her working as a sweets scientist, collaborating to create a large-scale pudding, aligning with her lifelong passion for understanding and innovating confectionery through science.