Rui Yashio attends Tsukinomori Girls' Academy as a second-year student in class 2-A, academically excelling at the top of her class. Her dry, logical, and results-oriented personality initially led her to dismiss music as meaningless. She speaks politely yet bluntly, adhering to a rigid daily schedule with minimal social engagement after school. Others often misinterpret her stoic demeanor as anger. She played violin from childhood but quit after placing fifth in a competition, concluding her extensive practice couldn't overcome a lack of innate talent. Fearing further disappointment, she abandoned competitive music entirely, shaping her avoidance of emotionally invested pursuits. She was childhood friends with Rinko Shirokane, bonding over music competitions and using the nickname "Rin-chan." Rui reacted with shock when Rinko withdrew from competitions. As a Tsukinomori student council member, she initially monitored Morfonica's band practices in an official capacity. Mashiro Kurata and other members persuaded her to join as violinist and composer. Her reluctance stemmed from believing music required unattainable talent, but gradual exposure to the band's sincerity shifted her perspective. She later became student council president to protect students from restrictive policies. When the new principal banned non-academic activities, Rui publicly challenged the decision, citing discord and confusion, later admitting her bandmates influenced this change. Her development includes prioritizing emotions alongside logic. She defends bandmates against threats, like intervening when students tried seizing Morfonica's practice room. Though initially opposed to deep connections, she befriends contrasting personalities like Kaoru Seta and Masuki Sato through art and music appreciation. She practices violin rigorously—4–5 hours on weekdays and 10 hours on weekends—retaining this habit from competition days. She addresses everyone by last name with "-san," including adults like Marina Tsukishima. She appeared in the spin-off series *BanG Dream! Girls Band Party!☆PICO ~Ohmori~*. Her hobbies are officially listed as "none," reflecting her past disinterest in non-academic pursuits. She favors zenzai soup with mochi but dislikes rich or strongly flavored foods. Physically, she has dark brown chin-length hair, thin brownish-red eyes, and a mature appearance. At 169 cm, she is Morfonica's tallest member.