Azusa Nakano is a central character introduced as the youngest member of the Sakuragaoka High School Light Music Club. She is a first-year student when she first appears, joining the group after being captivated by their performance at the school's freshman reception. She becomes the band's rhythm guitarist, playing a signature Fender Mustang guitar she later nicknames Muttan, and has been playing the instrument since the fourth grade, influenced by her parents who were members of a jazz band.

Physically, Azusa is the shortest member of the group with long black hair, often styled into two pigtails, and hazel eyes. She tans extremely easily, to the point where a brief exposure to the sun can dramatically change her appearance, leading friends to not recognize her. Her birthday is November 11th.

Azusa is defined by her earnest and serious personality, which stands in stark contrast to the lazy, tea-and-sweets-filled afternoons that dominate the Light Music Club's activities. She joined the club with high hopes for rigorous practice and musical dedication, only to find her expectations confounded by the senior members' lackadaisical attitude. This initial disappointment is so strong that she briefly considers quitting the club. However, despite her frequent exasperation and her complaints about the club's relaxed atmosphere, Azusa is not immune to its charms. She has a hidden weakness for sweets, especially cake, and is easily calmed down, sometimes simply by being patted on the head. Over time, she grows genuinely fond of the group's unique dynamic and, much to her own occasional shock, finds herself adapting to their slow pace.

Azusa's role in the story is that of a grounding force and a catalyst for growth. Her technical skill on the guitar surpasses that of Yui Hirasawa, who she initially admired as a talented lead guitarist. This leads to Azusa teaching Yui various techniques, pushing the band to improve musically. Despite her own proficiency, she remains modest, often referring to herself as a novice. She takes her responsibilities seriously, which eventually leads her to become the club president after the four original members graduate.

Her key relationships shape much of her experience. She is the primary target of Yui's affection, often enduring spontaneous hugs, which she pretends to dislike. Yui also gives her the enduring nickname Azunyan after convincing her to wear cat ears and meow, a moniker that sticks despite Azusa’s protests about being treated like a pet. Azusa deeply admires Mio Akiyama, seeing her as mature and an ideal, experienced bassist, and even attempts to give her chocolate on Valentine's Day. She also finds Tsumugi Kotobuki beautiful and, in their time alone together, teaches her how to play guitar. Her closest friendships among her classmates are with Ui Hirasawa and Jun Suzuki, with whom she often spends time.

Throughout the series, Azusa undergoes significant development. She transitions from an outsider frustrated by the club's inefficiency to its heart, dreading the day her senior members will graduate and leave her behind. Her fear of being alone is a powerful motivation, leading to an emotional plea for them not to leave. After their graduation, she steps into the role of club president, forming a new band called the Wakaba Girls with Ui, Jun, and two new members to continue the Light Music Club's legacy. Her notable abilities include her high level of skill and technical knowledge of the guitar, a structured and intelligent mind that helps her plan activities like study schedules for Yui and the graduation trip to London, and a hidden capacity for passionate, eruptive emotion that belies her usually calm demeanor.