|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, making her the third member.