Kotarō Azumi is the male protagonist of Tsukigakirei. He is a third-year student at Kawagoe Municipal Third Middle School and serves as the president of the Literature Club. He was born and raised in Kawagoe as an only child, living in a house built by his grandparents. His father was born, raised, and works in the same local area, while his mother is a housewife. Since childhood, he has been heavily involved in his local community, participating in a shrine's festival music group where he learned to play the flute and drums.

Kotaro is a quiet, reserved, and bookish teenager. He is a passionate reader who dreams of becoming a novelist and is deeply inspired by classic authors such as Osamu Dazai and Natsume Soseki. He spends many late nights writing his own stories, but he is initially too shy and embarrassed to share his work with anyone. He often appears calm and easygoing, which makes him seem detached from his surroundings, but internally he is thoughtful and introspective. He is not particularly proactive in his studies, resulting in grades that place him in the lower half of his class, and he does not typically stand out among his peers. He has a nervous habit of shadowboxing when he feels anxious or excited.

His primary motivation is his aspiration to become a professional writer. This passion defines his identity and drives much of his private effort. As his relationship with Akane Mizuno develops, his desire to be with her becomes an equally powerful motivation, pushing him to move beyond his comfort zone, express his emotions, and plan seriously for the future.

Kotaro serves as the emotional anchor of the story. The narrative centers on his tender and awkward first romance with Akane Mizuno. Their relationship begins after he is placed in the same class as her, and it slowly deepens through shy glances, mutual support, and long LINE message conversations. He supports her dedication to track and field, and she encourages his writing. His key relationships include his two best friends: the eccentric and playful Roman, and the grounded and straightforward Daichi. He also looks up to Daisuke Tachibana, an older neighbor and son of a local bookstore owner, who serves as a literary mentor and a trusted confidant. Kotaro views Takumi Hira, the athletic and popular track club president, as a romantic rival, though they maintain a respectful distance. His relationship with his parents provides another layer of conflict, particularly with his mother, who worries about his future and pressures him to focus on his studies.

Over the course of the series, Kotaro undergoes significant development. He grows from a painfully shy boy who hides his writing into a more confident and decisive young man. He learns to overcome his embarrassment, begins showing his novels to those he trusts, and submits his work for publication. He chooses to openly declare his relationship with Akane. When faced with her family's relocation to Chiba, he makes the determined decision to change his academic path and study intensely so he can attend high school near her. The series concludes with Kotaro having achieved his dream of becoming a published author, showing that his growth and dedication have carried him into a fulfilling adult life as a husband and father. His notable abilities are his talent and persistence in writing, as well as his skill in traditional Japanese festival music performance.