Umiri Yahata is a first-year student at Hanasakigawa Girls' High School and bassist for Ave Mujica under the stage name Timoris, managing the band's schedules alongside classmates Taki Shiina and Uika Misumi. Her shoulder-length black hair spikes slightly at the ends, featuring a straight-cut fringe angled left, with angular turquoise eyes and upturned eyebrows. Casually, she dons a black leather jacket, red crop top, short gray skirt with a black belt, and black laced platform boots. As Timoris, she performs in a lace mask adorned with red spider lilies, an off-white button-up shirt, black vest, red gold-lined shawl, and dark-cyan gem lapel pin, paired with black gloves, buttoned black-and-off-white leggings, and high-heeled boots.

Childhood betrayal shaped her: after leading an all-girls band to an audition, her bandmates abandoned her onstage. This trauma fuels her avoidance of deep band attachments, leading her to juggle multiple bands simultaneously to prevent recurring pain, alongside a fear of being seen as untrustworthy. Stoic yet polite in speech, she displays playful tendencies—balancing juice boxes on Taki's head or teasing her—and buys snacks when sensing Taki's distress. She fiercely guards close bonds, once semi-seriously threatening Uika with legal action for perceived disloyalty. Professionally, she is an exceptional bassist who supported 30 bands (with ten active), often performing for multiple groups daily, while neglecting meals for energy bars, drinks, and protein supplements.

In Ave Mujica, her alias Timoris originates from "Lacus Timoris" (Lake of Fear), aligning with lunar-themed stage names. Initially viewing the band as transient work due to past wounds, her perspective shifts after witnessing CRYCHIC's closeness; jealousy sparks a vow to reunite Ave Mujica post-conflict, signaling deepening investment. She exclusively uses a 5-string bass, distinguishing her from 4-string players in the franchise. She resides alone in a cramped apartment, reflecting her self-reliant isolation.