Kotarou Azumi is the main male protagonist of Tsukigakirei. He is a fourteen-year-old third-year student at Kawagoe Municipal Third Middle School, where he serves as the president of the literature club. He has a deep passion for literature and aspires to become a novelist, drawing inspiration from Japanese authors such as Osamu Dazai and Natsume Soseki. Kotarou is quiet and reserved, a stark contrast to his more outgoing male classmates, and he tends to be introspective and philosophical, often lost in thought about writing and his future. His upbringing is culturally rich: from a young age, he has been involved with his local Shinto shrine, learning to play the flute, taiko drums, and perform traditional dance, which has instilled a strong connection to his neighborhood. He is an only child living in the house built by his grandparents, and his family consists of a father who works locally and a mother who is a housewife. Academically, Kotarou is not particularly proactive; he ranks in the bottom half of his class and does not stand out among his peers, but he works part-time at the shrine to support his literary interests.

Kotarou’s role in the story centers on his developing romance with classmate Akane Mizuno, which begins when they are placed in the same class for their final year of middle school. Initially strangers, they grow close through shy, tentative interactions and frequent communication via text messaging. As the relationship progresses, Kotarou faces internal and external challenges: he struggles with the pressure of high school entrance exams, his father’s expectations, and his own insecurities. He also competes with Takumi Hira, a popular athlete who harbors feelings for Akane, but maintains a respectful dynamic with him. Kotarou’s personal development is marked by his evolution from a boy who hides his writing and emotions to someone willing to take bold actions to preserve his relationship. He publishes an online novel under a pen name, publicly declaring his love for Akane, and ultimately decides to apply to a high school near Akane’s new home in Chiba after her family moves, demonstrating his commitment to continuing their bond. The story ends with Kotarou and Akane married with a child.

Key relationships include his deep bond with Akane Mizuno, which drives the narrative; his friendships with Roman Yamashina and Daichi Ogasawara, who offer support and levity; his rivalry with Takumi Hira; and the brief tension with classmate Chinatsu Nishio, who develops a crush on him. Kotarou’s family, while present, remain more in the background. Notable abilities include his talent for writing and storytelling, as well as skills in traditional performing arts such as flute, taiko drumming, and dance, which he acquired through his shrine activities and which occasionally serve as points of connection with others.