Mio and Ritsu are childhood friends, but the latter could not remember how they first met. In elementary school, Ritsu often picked on Mio due to Mio being very shy, as well as left-handed and having silky black hair, among other things.Their first 'serious' interaction came when Mio won a prefectural award for an essay she wrote and she had to read it aloud in front of the school. Mio was depressed because of her shyness, so Ritsu encouraged Mio to imagine the audience as pineapples, which allowed Mio to get over her stage fright. Since then, the two have been best friends despite having very different personalities. Ritsu has loved to tease and scare Mio since they were kids, especially about scary things like ghost stories or disgusting ones like barnacles.
Ritsu also clings onto Mio like a lifeline when it comes to studies, and claims that Mio is the best tutor she has ever met. There were times when Mio clings onto Ritsu instead, when Mio was being bullied by Sawako or other people, hoping Ritsu would save her (but sometimes Ritsu joined in the bullying too).
They both also have a similar interest in photography, as Ritsu always brings a digital camera around, and Mio brings a film camera around. Ritsu was also the one who introduced music to Mio, and encouraged her to learn bass.
Mio and Ritsu understood each other so well that they could easily imitate each other, which helped them when they had to act as Romeo and Juliet, respectively, for their school's festival performance.
There were times when Ritsu got jealous when Mio was spending more time hanging out with others than with her, resulting in a brief conflict between them. However, such tension between the two does not last long.
During Mio's Fan Club Tea Party presentation, their other band members questioned if the slide show was really about Mio, since most of the pictures were dominated by Ritsu. Ritsu said that the photos were from her collection, which she had with Mio since they were little, therefore Mio appeared in every single photo. The photos showed how they grew up together as best friends.
Azusa has always tried to impress Mio and often goes with Mio's ideas since they are the two "sane" members of the light music club. She looks up to Mio because of her maturity, her bass guitar skills, and her more serious attitude than the other band members. When Mio and Azusa were alone together in the club room at one time, Azusa felt motivated to start practicing immediately, since she knew that Mio was the only one that would listen to her.
Azusa sometimes unintentionally makes remarks about Mio's weaknesses, such as her weight.
Yui and Mio are often known for not being on the same side, since Mio's maturity is not a good complement to Yui's childish personality. Yui often fools around and does not concentrate on practicing, 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 was too tough for a beginner such as her.
Mio is often jealous of Yui for having a fast metabolism. Mio is very concerned about her weight and tries not to eat anything that will increase it. Yui, on the other hand, eats everything, but never has any increase in weight, making Mio annoyed and jealous.
Despite their differences, Yui has often impressed Mio with her ability to learn things very quickly. And during their band's first concert, when Mio had 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.
Mio first met Mugi when Mugi was looking for the choir club. Mugi later joined the light music club only after witnessing the "fun and painful interaction" between Mio and Ritsu. Mio and Mugi have generally been good friends, but sometimes Mugi 'betrays' Mio, such as when taking vote over whether to play at the beach or to practice, where Mugi decided to play with both Ritsu and Yui.
Both Mio and Mugi are worried about their weight, which becomes a running joke once they find out that they share this concern.