|Tsumugi Kotobuki is the only daughter of the wealthy, respectable Kotobuki family. From her early childhood, she was much-treasured by her family and received special education. For instance, she began taking piano lessons when she was four, and as a precocious talent, won prizes at piano-playing competitions. However, due to her unique formative environment, she was isolated from other children her age. From thence Tsumugi developed a strong interest in living out a "normal" or "ordinary" life, as well as a moderate aversion towards discussing her family's prosperity.|
At one point, the Saitō family that had long served the Kotobuki household adopted a daughter, Sumire Saitō. Sumire quickly became Tsumugi's only playmate, and the two grew close, akin as siblings. Sumire often helped Tsumugi in "understanding" the lives of "ordinary" girls, for instance, by bootlegging manga for her, material that was forbidden to her. Tsumugi became enraptured with the concept of yuri, a genre involving love between women in manga, anime, and related Japanese media. It was probably due to the manga she read that she developed character traits that led many of her future friends to suspect her of being homosexual.
When Sumire turned twelve, she was enlightened about her actual relationship with the Kotobuki family. Because she was unwilling to follow the strict readjustments, a rebellious Tsumugi told her that she also planned to disregard her family's recommendations, and proposed running away together. After hearing that, Sumire changed her mind and persuaded Tsumugi to follow their families' instructions until Tsumugi could attend university.
Tsumugi eventually applied to Sakuragaoka High School, which her parents had recommended, and was accepted.
After entering high school, Tsumugi wanted to join the Chorus Club, and while checking the music room, met Ritsu Tainaka and Mio Akiyama who were looking for new members to re-establish the Light Music Club. Mugi was initially hesitant to join, but after seeing close, friendly interaction between Mio and Ritsu, agreed to enter the Light Music Club.
Afterwards, they went to a fast food restaurant where they discussed ways of attracting more members. Mugi was especially ecstatic at her first ever visit to a fast food joint.A few days later, Mugi, Ritsu, and Mio had placed posters of their club on school grounds, which attracted the interest of a new member, Yui Hirasawa, whom they were all eager to meet.They first thought of Yui as a guitar prodigy; however, they later learned that not only couldn't she play the guitar, but only knew how to play the castanets. Despite this revelation, they happily accepted Yui into their club to prevent it from disbanding.
After accepting new member, Yui Hirasawa, into their club, Tsumugi prepares tea for the entire group just as Yui arrives to the club room. Afterwards, the group decides to practice until they realize that Yui still doesn't have an instrument at which they all offer to accompany her the next day to help her find and buy a guitar.
Visiting a music store, Yui was able to find a guitar she becomes immediate attached to, but notice the price tag on the guitar indicates that it is too expensive. Mugi then agrees that they all get part-time jobs to help Yui get the guitar she wants, with Mugi expressing excitement over it. The group decides to take up a job by counting passing vehicles and passengers for a traffic survey, with Tsumugi showing visible enjoyment of it, while the others only show boredom and exhaustion, mostly Ritsu and Yui.After they all get their pay, Mugi gives her to Yui, but the latter declines, expressing how she had decided to buy a cheaper one out of guilt of having to rely on them for her own gain. The next day, she and the others return to the music store to find a cheaper guitar for Yui, but upon noticing Yui being upset of not being able to get the guitar she wanted, Mugi takes action and haggles one of the store employees, who then recognizes her as the store owner's daughter, thus allowing the price to be lowered to an affordable amount.
Yui then expresses her gratitude to Mugi. The very next day, Tsumugi and the others decide to make their club's goal be to perform at Budokan before they graduate and to celebrate the club's official formation and new goal, she and the others take a group photo.
Upon starting her new high school life, Tsumugi Kotobuki immediately went to check out each of the club, particularly choosing the check out the Choir Club.When she gets the school faculty, she asks any of the teachers if she could check the choir club, but is immediately asked by a girl to join them in the Light Music Club, which Tsumugi is seemingly a bit hesitant to do. However, upon seeing how the girl's friend scolds her for trying to force Tsumugi to join, she decides to join their club upon sensing how fun it may be, thus making her the third member.
Some time later, she and the other club members receive an application form from a girl named Yui Hirasawa who is about to join their club and are all happily awaiting her arrival. When Ritsu encounters Yui on the way to the club room, Mugi prepares some tea for her and begin inquiring Yui why she wanted to join the club. Unfortunately, they learn that Yui was now intending to quit the club ahead of time, but Mugi, Ritsu and Mio play a bit of music they just made for her to help her make a choice of either joining them or not. Although given brutal honesty about their performance, it is enough to have Yui decide to officially join the club as their fourth member, therefore officially starting the Light Music Club.
The following day, Tsumugi has prepared tea for all of the club members and she unknowingly astonishes Yui when she reveals to her that the tea set they are using are ones that she brought from her house. Unfortunately, she and the other members learn that Yui still doesn't have an instrument to play, specifically a guitar due to her role as the club's guitarist. Luckily, Mugi and the other members agree to accompany Yui to a music store so they can help her buy one. The next day, Mugi meets up with Mio and Ritsu before they are joined by Yui. The four of them head to the music store where Mugi finds that Yui has quickly taken a liking to one of the guitars, but is unfortunately too expensive for her to buy. Wanting to help her new friend out, Tsumugi asks one of the store employees if they can lower the price of the guitar Yui is eyeing. Fortunately, the employee she asked recognizes her as the store owner's daughter and quickly complies to her request and Mugi immediately tells her friends about the good news.
The very next day, Mugi, along with Ritsu and Mio, cheer for Yui as she shows off her newly bought guitar to them. Mugi is then asked by Yui where she gets all the sweets she brings to the clubroom from, which Mugi answers by telling her that her family gets various gifts of sweets so bringing some to the clubroom everyday isn't a bother as they always have too much at home, leaving Yui to think why her family gets so much from others. The following day, Mugi has finished taking her exams and visits the clubroom where everyone else has just arrived after their own exams. Mugi expresses how difficult the exams have gotten lately, but she learns that out of the four of them, only Yui failed. Mugi offers her some word of encouragement to help her in her studies for the make-up exams. Later, Mugi is invited to Yui's house, where Mio will hold a study session to help prepare Yui for the make-up exam she will take. The very next day, Mugi and the rest celebrate Yui's perfect score on her make-up exam.