Chiya Ujimatsu serves as a central character across the franchise's multiple seasons and specials. Her family owns and operates the traditional Japanese sweets café Ama Usa An, featuring a rabbit mascot named Anko. Historically a business rival to Rabbit House, run by Chino Kafū's family, this rivalry remains dormant between their granddaughters. Chiya's name holds thematic weight, referencing Uji matcha green tea from Kyoto's Uji region and signifying "a thousand nights" through its kanji.

Physically, Chiya possesses long, straightened dark brown hair styled in a hime cut with distinctive forelocks, fair skin, and downward-curving green eyes. Her typical work uniform is a dark green vertically striped kimono paired with a long white frilly apron. She displays an uncanny ability to dodge objects thrown near her. Personality-wise, Chiya presents as gentle, soft-spoken, and formal, often acting as a peacekeeper within her friend group. She maintains a consistently light and airy demeanor, smiling even when upset. Underneath this calm exterior lie distinct quirks: she enjoys assigning elaborate poetic names to ordinary sweets (e.g., "Red Jewel in a Snowy Field" and "Moon and Stars Reflected Upon the Sea"), tells scary stories to unsettle friends, harbors a dream of becoming a samurai, and exhibits a playful, teasing side, particularly toward her childhood friend Syaro Kirima.

Her background establishes a long-standing friendship with Syaro, dating back to early childhood. Chiya is the sole person privy to Syaro's true impoverished background and residence, facilitated by their adjacent workplaces. She deeply values their bond and hopes Syaro will eventually work with her at Ama Usa An, feeling genuine hurt when Syaro initially expresses no such intention. Within the broader friend group—including Cocoa Hoto, Chino Kafū, Rize Tedeza, Maya Jōga, and Megumi Natsu—Chiya often assumes an older sister role due to her relative maturity and tendency to offer guidance.

Character development unfolds through her interactions and actions. In the *Dear My Sister* special, Chiya proactively organizes a group outing to a summer festival to watch fireworks, securing yukatas for everyone to enhance the experience, demonstrating her role in fostering group cohesion. Her subtle disappointment upon Cocoa's temporary departure reveals underlying emotional investment in their friendships. Later storylines, including those in *BLOOM*, show Chiya participating in seasonal traditions like preparing Galette des Rois for New Year's celebrations and supporting Chino's evolving aspirations for broader experiences beyond their town. These moments reinforce her consistent presence as a stabilizing and caring figure within the group dynamic.