Dai Miyamoto is the central protagonist of Blue Giant. He begins his journey as a high school student in Sendai, Japan, where he initially pursues basketball. Feeling limited by his physical abilities in the sport, he shifts his focus entirely to jazz after a friend introduces him to a concert, an experience that leaves him stunned by the music's raw passion and intensity. From that point forward, he devotes himself to mastering the tenor saxophone with the singular, ambitious goal of becoming the best jazz player in the world. His background includes a family consisting of his father, an older brother, a younger sister, and a mother who passed away when he was young.

In terms of personality, Dai is characterized by an unshakeable cheerfulness and an extremely outgoing, positive nature. He possesses a straightforward, loud, and honest demeanor, often speaking his mind without hesitation. He is remarkably resilient, able to shake off insults and criticism from others with ease, and he maintains an unwavering belief in his dream regardless of how many people doubt him. A recurring phrase of his in times of hardship is "This ain't nothing," reflecting his refusal to give up. While he cares deeply for his friends and family, he takes music, especially jazz, very seriously, almost to the point of obsession. He often prioritizes his musical pursuits over normal relationships, which can strain personal bonds. This immense dedication is a major reason why his family, teacher, and friends ultimately support his career aspirations.

Dai's primary motivation is his burning desire to become the world's greatest jazz musician. This goal drives every action he takes, from practicing for hours on end, often in harsh conditions, to leaving his hometown to pursue professional opportunities. His role in the story is as the driving force and heart of the narrative, a self-taught genius whose raw talent and relentless energy inspire those around him. After graduating high school, he moves to Tokyo to chase his dream, forming the jazz band JASS with two key partners: Yukinori Sawabe, a gifted and disciplined pianist, and Shunji Tamada, Dai's friend from Sendai who is a complete beginner on the drums. These relationships are central to his development, as the trio navigates the challenges of the music industry together, aiming to perform at So Blue, Tokyo's most prestigious jazz club. Other key relationships include his mentor Yui, a strict teacher who helps him rapidly improve his technique in his final six months of high school.

Throughout the story, Dai undergoes significant development. He evolves from a completely self-taught musician with raw, unrefined power into a more skilled and polished player, thanks to formal training and the collaborative experience of playing in a band. His style remains distinctive for its overwhelming intensity and passionate sound, which has the ability to move audiences even when his technique is imperfect. After achieving success in Japan, his journey continues as he travels to Europe and the United States to pursue his dream on an international stage. Dai's most notable abilities are his powerful, passionate saxophone playing and immense stamina. His powerful sound is often described as being able to "overwhelm" a listener, a quality partly derived from the physical conditioning and breath control he developed as a basketball player. He is also portrayed as a natural genius, possessing a level of technical prowess, velvety tone, and musical creativity that would normally be unattainable for someone who has only been playing for a few years, showcasing both his extreme dedication and innate talent.