Ui is Yui's younger sister, who begins the story as a third-year junior high school student but later enters Yui's high school the following year in the same class as Azusa.

Unlike her older sister, Ui is mature, responsible and handles household chores well. Ui also shares a strong relationship with Yui and has a great deal of love and respect for her older sister. She takes particularly good care of Yui and strives to look after her even at the risk of her own health.

Despite being a year younger than Yui, she is nearly identical to her with her hair down and is even once able to fool the club. She eventually joins the Light Music Club along with Jun at the end of the series, becoming a guitarist like her sister.

Source:Wikipedia