|Ui was born one year after her older sister Yui Hirasawa and grew up with her together, developing a strong affection to her which was returned equally. One christmas in the past, she wished for a "White Christmas" which was provisorily granted the next day by Yui who stuffed a bush in front of their house with the foam of a pillow which earned Yui heavy scoldings from their father. Since their parents often travel across the world, Ui got used to fill the role of the household's caretaker and constantly looked after her older sister. Eventually, she became friends with Nodoka Manabe who often visited Yui at home and also build a friendly connection to their neighbour Tomi Ichimonji. After graduating from elementary school, she started visiting the same school Yui and Nodoka attended and became friends with her schoolmate, Jun Suzuki.|
|On the first day of Yui's high school career, Ui wakes her sister up. Yui got the time wrong and starts rushing towards her new school, leaving a confused Ui behind.|
She later gives her sister and her friends some snacks while they help with Yui's studies. When Ritsu is told to go outside of the room due to her messing around, she plays video games with Ui to pass the time, with Ui always being the winner.