Ui Hirasawa (平沢(ひらさわ) 憂(うい)) is Yui Hirasawa's younger sister. Unlike her sister, she is a more mature person and handles household chores easily.
Ui's parents are a dating couple and are rarely home, so she does house tasks in their stead. When the story begins, Ui is a third-year junior high school student, but later she enters the same school as her older sister in high school the following year. Unlike her older sister, Ui is mature, smart, responsible and handles household chores well, but she has a great deal of love and respect for her older sister and takes particularly good care of Yui, striving to look after her even at the risk of her own health.#1 She has an extreme talent for playing the guitar#2, and became capable of playing with the rest of the light music club band only after a few days of practice. Ui is actually considered as the ultimate groupie for her sister's band and supports them with all her heart. She occasionally provides a narrative to the story. She is in the same class as Azusa and hangs out with her sometimes.
She is a fast learner like Yui, as she was once at a baseball center with Jun and Azusa, and after Ui overheard some tips from a stranger, she followed his tips and managed to score a homerun on her first at-bat.
Sometimes, she teaches Yui how to play some parts of the guitar since Yui is not so good at reading sheet music. Despite being a year younger than Yui, she looks nearly identical to her with her hair down. Once, she pretended to be Yui when her sister was sick but failed because she did not know the nicknames Yui had given to the light music club members and Sawako said Ui's breasts were bigger.
Ui and Yui look like twins but they're not the same age. One time Ui pretended to be Yui because she wanted to try to hug Azusa.
She joins the light music club along with Jun at the end of the series.