Yui and Mio are often known for not being on the same side. Mio's maturity often doesn't work with Yui's childish personality. Yui tends to only want to play and keeps forgetting to practice, which usually makes Mio annoyed.

During their first meeting, Mio taught Yui the basics of playing guitar and the basic guitar chords, which Yui thought it were too tough for a beginner like her.

Mio even often got jealous of Yui for having a fast metabolism. Mio is very concerned about her weight and tries  not to eat anything that will increase her weight. On the other hand, Yui eats everything, but never has any increase in weight, making Mio annoyed and jealous.

Despite this, Yui has managed to impress Mio with her ability to learn things quickly. And during their band's first concert, when Mio was supposed to sing but was suffering from stage fright, Yui told Mio that she believed in her and knew that Mio had been practicing the song. The other band members followed suit, and the performance was a success.