Mio Akiyama is a student at Sakuragaoka High School who initially intended to join the literature club but was persuaded by her childhood friend Ritsu Tainaka to join the Light Music Club. She serves as the bassist, second vocalist, and primary lyricist for the band Ho-kago Tea Time. Her personality is defined by a stark contrast between her reserved, shy nature and her deep inner warmth and kindness. She is extremely self-conscious and dislikes being the center of attention, often experiencing stage fright and anxiety when performing in front of others. This shyness is frequently a source of comedy, especially when she is teased by her friends or forced into embarrassing situations. Despite her timidity, she possesses a strong sense of responsibility and often functions as the voice of reason within the group, guiding her more carefree bandmates with her practical and sensible approach. She is academically excellent, diligent, and takes her musical role very seriously, pouring genuine emotion and personal experience into the lyrics she writes.

Mio is primarily motivated by her dedication to music and her deep affection for her friends. She works hard to improve her technical skills on the bass guitar and to ensure the band's success, seeing her commitment to the Light Music Club as something akin to a devoted relationship. Her songwriting is a key outlet for her feelings, often resulting in lyrics that are unexpectedly sentimental and girlish. Within the story, she acts as a stabilizing force, balancing the energetic and often chaotic personalities of her bandmates. She is also the target of much of the group's affectionate teasing, particularly from Ritsu, and serves as the straight man for many of their antics. Her role in the band is musically indispensable, as she provides the low-end foundation and writes the majority of their original songs.

Her key relationships are central to her character. Her bond with Ritsu is the most significant; they have been friends since childhood, and while Ritsu's playful provocations frequently frustrate Mio, their friendship is one of deep mutual trust and affection. With Yui Hirasawa, Mio acts as a responsible and guiding figure, often helping the carefree guitarist focus. She shares a calm and supportive rapport with Tsumugi Kotobuki, who she often collaborates with on creative ideas. With the younger Azusa Nakano, Mio takes on a protective, big-sister-like role, offering guidance and understanding that stems from her own experiences with anxiety and dedication.

Mio undergoes noticeable development over the course of the series. She gradually learns to manage her stage fright and becomes more comfortable with her role as a performer. A key moment occurs during her first school festival performance where an embarrassing fall paradoxically endears her to the audience, helping her relax and embrace her identity as an artist. She matures in her confidence as a lyricist and musician, learning to accept praise and stand her ground when necessary. By the end, she remains fundamentally shy but has grown into a more assured leader and a more openly playful and cheerful friend, having overcome many of her initial insecurities through the support of her band.

Her most notable abilities are her high level of technical proficiency on the left-handed bass guitar, her clear and emotive singing voice, her talent for lyrical composition, and her strong academic record. She is also surprisingly physically capable in stressful situations involving her friends, though she remains extremely afraid of ghosts, scary stories, and gory topics.