Always looking for fun, Yui is fearless; anxiety and stage fright are unknown to her. She likes everything cute and makes a lot of physical contact like holding hands or hugging. She also loves everything sweet (that's why Ritsu gave her a Alias "Lazy Fairy"). Sweets that Mugi brings are Yui's power source. Her physical constitution is such that she does not gain weight no matter how much she eats. She can't stand being hot or cold, so she spends the winter under the kotatsu (a knee-height table with an electric foot-warmer installed inside on the top board, which is used with a coverlet during winter)[1] and rolls around on the floor in the summer in front of the fan, to keep cool.[2] Her grades are always on the verge of failing exept for the fine arts in which she is exceeding; sometimes she is the only person in her class who needs to retake a test[3], other times she manages to avoid failing all of her subjects by a one point margin. She has an average figure. Yui lives in a family of four with her younger sister Ui and both parents. Her parents are a "lovey-dovey" couple[4] and often go on trips together, leaving both daughters behind.

Main character, lead guitarist, lead vocalist. Having misunderstood that "light music" equals to "easy music" (like whistling), she joined the Light Music Club and picked up guitar with absolutely no prior experience.[5] She cannot do more than one thing at a time, but to make up for that she can be highly focused on a particular subject and once started, can advance quite sustantially. However, whenever she learns something, she forgets something else.[6] Yui is the type of person who doesn't read manuals and relies on her own intuition and self-taught approaches. She also does not remember any of the musical terminology. She possesses perfect pitch and is musicaly gifted. She is able to tune her guitar without a tuner[7], but she does not appear to make the best of her ability. Yui is relatively determined and competitive, but at the same time she feels comfortable depending on the help of a few good people like Ui, Nodoka, Mio and Azusa.