A junior one year below Yui Hirasawa and the others. She first appears in Volume 2 of the manga and Episode 8 of the first season of the anime. She is characterized by her black twin-tails. Her parents play in a jazz band, and she has been playing the guitar since she was in the fourth grade. Among the characters, she has a serious and sensible personality, which often leads to her being swept up in the antics of others. As she is the first junior member, she is doted on by Yui, and she is also close to Yui's sister Ui, who is in the same grade as her. On the other hand, she has a straightforward and childish side, such as being teased by Ritsu, being stubborn, or directly expressing her desire to actually practice during club hours. She is also kind-hearted, looking out for and taking care of Yui and the others. She possesses solid guitar skills and sometimes teaches Yui, who lacks basic knowledge. Because of how she looked wearing cat ears, Yui gave her the nickname "Azu-nyan." This name is used affectionately not only by Yui but also by fans and the voice acting cast, including Ayana Taketatsu, who voices her. She seems to have a complexion that tans easily; even with sunscreen, she ends up getting dark, leading to the nickname "Koge-nyan" (Scorched-nyan).

Ritsu Tainaka's younger brother, who first appeared in Episode 13 (Extra) of the first season of the anime. He is currently a middle school student. He gets along well with his older sister, Ritsu; they go to the movies together and engage in light sibling bickering. He also seems to be acquainted with Mio Akiyama, who has been visiting the Tainaka household since they were young, and Mio addresses him by his first name without honorifics. In Episode 16 of the second season, he showed typical adolescent behavior by hiding in the bathroom when the Light Music Club members came over to visit. In Episode 18, when he heard Ritsu was playing the heroine Juliet in the class play for the school festival, he spat out the cola he was drinking and burst into laughter. Incidentally, a friend who seems to be from the same middle school appears in Episode 13 of the first season; both are among the few male characters in the series. He seems to be very well-loved by the anime staff and cast, especially Satomi Sato, who voices Ritsu; cheers often erupted during his scenes in the disc commentaries. He also makes an appearance in the opening and the final stages of K-ON! Movie.

She plays the bass and also serves as the vocalist for certain songs. When she first entered Sakuragaoka High School, she intended to join the Literary Club. However, she was forced into the nearly defunct Light Music Club by her childhood friend Ritsu Tainaka, who entered the school with her. Along with Tsumugi Kotobuki, they started the club. This became the catalyst for awakening her dormant talent as a musician. In the story, she is one of the more sensible characters and excels academically, a trait she shares with Nodoka Manabe, making her someone other students rely on. On the other hand, she lacks self-confidence and is extremely shy (she reportedly chose the bass because she "didn't want to stand out"). She is very uncomfortable in situations where she is exposed to many people, such as live performances, and becomes extremely intimidated in such moments. Furthermore, despite her big-sisterly vibe and somewhat masculine speech patterns, she is easily lonely and dislikes painful or scary stories. Her masculine speech seems to be an influence from her childhood friend Ritsu; notably, when she gets scared, she reverts to feminine speech. Perhaps due to this personality, she often ends up being the butt of jokes or drawing the short straw in the group. It is usually her role to be forced into wearing the cosplay outfits handmade by the advisor, Sawako Yamanaka. Even after entering university, this personality hasn't changed; in contrast to Ritsu, who immediately hit it off with Ayame Yoshida and the upperclassmen, Mio struggled to become friends with Sachi Hayashi. She is also sometimes treated like a dress-up doll by Kana Yoshii and Ayame. As mentioned, her bond with Ritsu dates back to elementary school, which is why she is completely at ease around her. She often punishes Ritsu's pranks with a fist to the head, but as noted above, when she becomes intimidated, her childish side comes out, and she shows a cute side by clinging to Ritsu.

After entering Sakuragaoka High School, the setting of this work, she learned that the Light Music Club was on the verge of being abolished. She forcibly dragged her childhood friend and classmate Mio Akiyama into it to revive the club. She serves as the president of the Light Music Club. As such, she is often seen leading the other members. In the first volume of the manga, she spoke in a normal feminine tone, perhaps putting on an act to recruit Yui, though she returned to her usual self immediately after. She constantly teases the easily-frightened Mio. She is commonly called "Ricchan," or sometimes "Ricchan-taiin" (Member Ricchan) or just "Taiin" (a nickname also used by her fans). She has a lazy and disorganized personality, often forgetting to submit applications for the use of the auditorium, which leads to lectures from Mio or Nodoka Manabe. This personality once caused an unexpected and major incident that most readers never saw coming. Contrary to her personality and boyish appearance, she has high domestic skills on par with Ui Hirasawa (Yui's sister) and has a homemaker side. Perhaps because she takes care of her younger brother Satoshi, she is quite good at cooking and sewing (limited to hand-sewing; she is bad with machines), even with small, detailed tasks. While she is usually the one teasing Mio or Azusa, she tends to shrink back unexpectedly when she becomes the target of teasing herself. At times, she reveals a side that is even more delicate and timid than Mio's. Though they seem like opposites, she and Mio are surprisingly similar. While she stands out as a troublemaker, she is actually the most social member and the mood maker of the club who can read the room. She is clever in both good and bad ways, which allows her to sometimes ease Mio's tension and other times tease her relentlessly. She is surprisingly polite to her elders. The only exception is the advisor and friend Sawako Yamanaka, whom she addresses casually outside of class. Of course, she respects her both as a teacher and as a senior in the Light Music Club. Her sociability remained intact in university, where she befriended Ayame Yoshida and the upperclassmen very quickly. Among the seniors, she seems particularly close to Chiyo Hirose.