|During elementary school, Mio was an extremely shy girl who often read books. Her classmate Ritsu Tainaka became interested in her for some reasons and started to pick on her on a daily basis which often brought Mio on the verge of tears.
At some point, Mio won an award for a composition that she wrote at school, which meant that she had to read it in front of a large audience. Due to her timidness, she was really depressed about it until Ritsu found her alone on a playground. After she told her the problem, Ritsu promised her to help her train for the reading and brought her to her home.
Since she was even too shy to read it in front of a single person, Ritsu taught her to imagine the audience as pineapples to stay calm. The trick actually helped Mio to read her composition without problems. Since then, she continued to visit Ritsu at home and the two became best friends. Miraculously, the two could always visit the same school and class together. Since she often visited the Tainaka household, she got to know Ritsu's parents and her younger brother Satoshi Tainaka.
Years later, Ritsu and Mio watched a concert at TV which amazed Ritsu so much that she convinced Mio to form a band together. After Ritsu bought the sticks for her drum set, Mio struggled about what bass she should choose until she could finally decide to buy a "Fender Japan '62 Reissue Jazz Bass". After buying it, she highly cheriched it and was distraught when it got its first scratch.
After graduating from Junior High School, she was accepted into the Sakuragaoka High School.
After starting her high school career, Mio wanted to join the school's Literature Club. However, her plan was thwarted by Ritsu, who simply ripped the club application in front of her. Ritsu wanted Mio to join the Light Music Club together with her. After hearing that the club needs at least four members to be accepted, Mio was about to give up, just to be forced by Ritsu to search for new members together.
Later, she and Ritsu decided to wait for new members to arrive, but after a few moments, she told Ritsu that possibly no members are going to arrive and join the Light Music Club. Just then, another student had arrived, asking them if they were the Choir Club. When Ritsu tried to force this student to join them, she quickly scolded her and later, corrected her about the flashback she had about them watching a concert together. However, the student later introduces herself to them as Tsumugi Kotobuki, who, after finding both of them to be funny and interesting people, happily decided to join the Light Music Club.
Afterwards, she, Ritsu and Mugi had decided to meet up at a fastfood restaurant to discuss on how to get new members to join their club before it is disbanded. A couple of days later, she comes up with the idea of leaving leaflets on the school grounds in the hopes of getting at least a few students' attention and hopefully join their club.Later on, she and the others receive a club application form about a girl named Yui Hirasawa, whom she, Ritsu and Tsumugi, become eager to meet, imagining and expecting her to be a guitar prodigy. Just then, Ritsu comes into the music room with Yui herself and they all greet her. However, to their dismay they learn that Yui doesn't actually know how to play any instrument apart from castanets and, after listening to her reason for deciding to join the club, decide to play a short music the three of them made. She, Mugi and Ritsu are happy to accept Yui into the club for real, thus officially saving the club from being disbanded.
A day after accepting Yui as the new member of the club, Mio is asked by Yui on why she chose to play the bass guitar instead of some other instrument, where Mio gives her reason for choosing her role as bassist in the group.
Upon being asked by Yui to picture herself as the center of attention had she chose a different instrument, Mio faints immediately afterwards. After remembering that Yui still doesn't have an instrument to play yet, she and the others decide to help Yui buy one. Meeting up the next day, Mio, Ritsu, Mugi and Yui head over to a music shop to find Yui an instrument that she can play, with her having been attracted to a particular guitar, but they find out that Yui is unable to afford it due to its price.
Thinking of a way to help Yui, Mio and the rest of the club decide on getting part-time jobs. She and the four then take part-time jobs in counting automobiles and passengers for a traffic survey. They finish their shift and are given their pay, to which Mio and the others give to Yui, however, the latter decides to not take it, stating how she feels bad for having to rely on them for her own wish and decide to buy a cheaper guitar the next day.
The next day, Mio and the others return to the music store to buy a cheaper guitar, but to their surprise, Mugi had ended up haggling one of the shop's personnel, who recognizes her as the shop owner's daughter, who then lowers the Les Paul Yui had wanted to an affordable price. Mio and the others then express their happiness to Yui. Back at school, Mio hears Ritsu's goal for the club to perform at Budokan before they all graduate, with all of them taking a group photo to commemorate the club's new goal.
one of the teachers that the Light Music club will be disbanded unless four new members join to save it. Despite wanting to give up on starting the club, Mio is stopped by Ritsu again when she realizes that she may become the club's president if they join. Another student then comes in who Ritsu responds to by asking her to join their club. Mio reacts to this by telling her best friend to not force someone into joining them and scolds for making up a promise they did not not make. However, to Mio's surprise, the girl joins them anyway after noting that the club may be fun.
A few days later, Mio and the others get an application form from an upcoming new member, a girl named Yui Hirasawa, whom they are all awaiting the arrival of excitedly. When Ritsu later encounters Yui upon heading to the clubroom, Mio and Mugi prepare tea for her, happily welcoming Yui to the Light Music Club. Unfortunately, their joyous welcome is short-lived when Yui reveals that she was intending to quit the club, much to the three's dismay. Mio then joins in when Ritsu decides to hold a performance for Yui to help her decide if she wants to join their club or not. Despite Yui's brutal honesty that the performance wasn't all that good, it is enough to convince her to join their club at last, thus fulfilling the membership requirement to formally start the Light Music Club.
The next day, at the clubroom, Mio is asked by Yui why she chose to be the bassist. Mio merely says that being the guitarist is embarrassing, shocking Yui. Mio explains that the guitarist is the center of attention of a band, voicing out how she won't be able to handle all that admiration which causes her to faint on the thought of it. Mio then asks Yui if she's bought a guitar yet. After a moment, Yui remembers that she is the guitarist, meaning that she forgot to buy one. Mio is asked by Yui how much a guitar costs. Mio says that a cheap one would cost at least 10,000 yen, but voices out how cheap ones aren't naturally great and that she should buy one that's 30,000 yen or more, causing Yui to react over how expensive they are. The very next day, Mio, along with Ritsu and Mugi, accompany Yui to a local music store to help her buy a guitar. As Yui wonders what to choose, Mio expresses the many varieties she can buy and explains them each to her, but soon realizes that Yui isn't listening as she gets distracted by one guitar she has become attracted to.
Later the next day, Yui shows off her new guitar to Mio and the others, the one she was immediately attracted to the day before. Mio and the others cheer for her. However, Yui can't play the guitar yet as she hasn't practiced at home. Yui says that she couldn't think about playing her new guitar because she admires it so much, Mio asks if she hasn't removed her guitar's protective film yet and Yui nods to it until Ritsu quickly yanks it off, causing Mio to scold her and apologize to Yui. When Yui asks how she can play her guitar during a live performance, Mio points that she has to plug it in on an amp. Yui plays it, but she is still upset that she can't play any other notes yet. When she goes to unplug her guitar, Mio warns her to turn the amp's volume down first, but it is too late as Yui gets blasted by the loud wave of the amp. Mio later suggests that Yui study the chords, but the latter asks her confusingly to teach her how to read music, to Mio's shock.