Yui Osada is a very calm silent girl who like reading books, and helping others too. She is best friend with Muki since Middle School, and they hange around with each other a lot even after middle school too. She only gives silent treatment to other people she don't know, just like not talking to strangers.
At her younger age, she was a lonely girl who doesn't have friends around throughout years until she met Muki for the first time. She lives alone with her older sister, and spends time with her too. She mostly does her hobbies most of time, like reading books, sleeping, playing with her animal friends such as cats and dogs, and exploring bugs too. She mostly likes to listen to listen to her trusty old music box which her grandmother gave her too. She listens to it whenever it helps her relax, and keeps her happy too. This music box plays most of different songs too. She always has her music box for every year, and always keeps it within her heart.
Years later at Middle School. During lunch time at the cafeteria, she always puts on her headphones, to listen to the music box while eating so she don't disturb people. Then a random girl gets really irritated and annoyed at her always bringing that box with her, and threatening to break those box in pieces, a girl's friends tries to stop her saying it's not necessary, and worth it, but she goes to Yui anyway, and tries to take it from her. Just after she takes her music box, and about to slam it on the ground, that's where Muki Koizami comes, turns the girl around to her, and pushes her on the ground away from Yui. After the girl and her friends retreats, Yui looks at Muki, as Muki holds Music box to her, giving her a nice smile as Yui looking at her, and started to feel really cheerful for the very first time. After they introduced themselves, they became best friends and even after middle school too.
Day after High School started, she makes an appearance as a 1st year student, wearing her coat with red scarf, seeing Muki again at High School. Then as they get ready to start, Muki introduced Yui to her childhood friend, Niako Mazumi. Yui greeted him with a bow saying it's nice to meet you, hoping they can see each other more, then Niako gives her a nice smile, and greets her as well, making Muki very happy. She became smart, and addicted to science stuff too.
Months later at High School. She notices a new student Eluza Motoyashi coming by. She was wondering who she was, and had no choice stalking her, like every day. She also notices Muki trying to be friends with her, so she comes out, and tries to help her out.
After seeing her friends having powers, she was thinking of having one too to help them out. She started mixing potions all together, then started putting a bottle of mixed potion in the machine. As it finished, the machine opens up, creating an glowing orb, as the empty bottle drops out of the machine. She grabs an orb, looking at it. Then she gobbles down an orb, swallowing it whole, and notices a background place in different colors, like everything is light blue, and frozen in time. And that's how she gains an ability to stop time.