Mio and Ritsu are childhood friends, but the latter could not remember how they first met. Their first 'serious' interaction came when Ritsu encouraged Mio into imagining her as a pineapple and read out her essay in front of the school when they were still in elementary school. Ritsu really loves to tease and scare Mio since they were kids, especially about ghost stories or barnacles. However, the end results is always a fresh new bump on the drummer's head. Ritsu was first amused over Mio's silky black hair and Mio being a left-handed.

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. It was their promise that they would form a band together, as the bassist and the drummer, since they were little. Ritsu and Mio understood each other so well that they could easily imitate each other, and at the same time act as Romeo and Juliet for their school's festival performance. There were moments when Ritsu got jealous when Mio was spending more time hanging out with others than with her.

During Mio's Fan Club Tea Party presentation, their other band members questioned if the slide show was about Ritsu or Mio. Ritsu claimed that those were her photo collections she had with Mio since they were little, therefore she had appeared in every single photo. Those photos showed how they grew up together as best friends.

Azusa always tried to impress Mio, either by making her cake for Valentines Day (which she later found out was shared by everyone in the club) and always goes with Mio's idea. She is inspired by Mio because of her maturity and the fact that she is an experienced bassist. Mio also gives her comfort whenever she feels sad and shows care as if they are siblings (in fact, both of them look very similar to each other). She also often tries to change the way she looks, starting by undoing her ponytail and straightening her hair, which later revealed that she looks similar to Mio. At the same time, Azusa had always wanted to have a mature sister like Mio.

Despite all that, she sometimes unintentionally makes remarks concerning Mio's weaknesses, such as her weight.

Yui and Mio are often known for not being on the same side. Mio's maturity often don't fit Yui's childish personality. Yui oftenly only wants to play and keeps on forgetting how to practice, which makes Mio often annoyed.

During their first meeting, Mio teaches her how to play the basics of guitar and teaches her the basic chords in guitar, which again Yui thought was too tough for a beginner such as her.

Mio even often got jealous of Yui for having fast metabolism. Mio is very concerned of her weight and try to eat everything that won't increase her weight, while Yui eats everything, but never had any increase in weight, making Mio more annoyed and jealous. Despite these differences, they made a close relationship between each other.

Mio first met Mugi when Mugi originally intended to join the choir club. Mugi later joins Light Music Club only after witnessing the "fun and painful interaction" between the duo. Mio and Mugi have been good friends, but sometime Mio finds Mugi to 'betray' her, such as, when taking vote of either to play in the beach or to practice, Mugi decided to play with both Ritsu and Yui and Mio thinks she betrayed her (in kind form).

Both of them are also worried about their weight and became closer because of that.