Azuma Kazuki is the protagonist of the anime series BBK/BRNK: The Gentle Giants of the Galaxy. He is introduced as a teenage boy who spent the first part of his childhood on Treasure Island, a mysterious floating landmass far above Earth. This island was home to giant, sleeping, sentient robots known as Buranki. Azuma lived there with his mother, Migiwa Kazuki; his father; and his identical twin sister, Kaoruko. When the Buranki on the island began to awaken, his mother made the difficult decision to send Azuma, his sister, and his father down to the planet's surface inside a Buranki named Oubu to ensure their safety, leaving her behind on the island.

At the start of the story, ten years have passed since that separation. Azuma returns to a post-apocalyptic version of Shinjuku, a place vastly different from the world he left. His upbringing on Treasure Island left him completely unfamiliar with the concept of Bubuki, which are sentient weapons that form the individual limbs of a Buranki. Due to this ignorance and the chaotic state of the world, he is quickly captured by authorities. He is rescued by his childhood friend, Kogane Asabuki, who is a Bubuki user herself.

Azuma is defined by his kindhearted nature and a strong, unwavering sense of justice. He is earnest and determined, though his sheltered upbringing can make him naive about the workings of the world below. As the inheritor of Oubu's Heart, a Bubuki passed down from his mother, Azuma discovers his own power. His role within the group of Bubuki users is unique and vital: he is the Heart. This means he possesses the core component necessary to awaken and assemble the complete Buranki, Oubu. As the main controller of Oubu, Azuma is linked to the giant robot on a profound level. When Oubu suffers damage in battle, Azuma physically feels that pain, and the Buranki's performance is directly tied to Azuma's own emotional and physical condition. This connection makes him a powerful but vulnerable leader in combat.

His key relationships drive much of his development. His bond with Kogane Asabuki, who wields Oubu's Righty limb, is foundational; she is not only his rescuer and protector but also a steadfast ally who helps him navigate a dangerous new reality. A central and complex relationship develops with his twin sister, Kaoruko. After being separated for ten years, their reunion is far from warm. Kaoruko is hostile and dismissive of her brother, at one point even demanding that he hand over Oubu's heart. Their sibling conflict becomes a significant challenge for Azuma, forcing him to confront both a personal rival and the fractured pieces of his own past.

Throughout the story, Azuma shows considerable emotional development. He begins as someone largely protected from the world's harshness, but he is quickly thrust into a position of great responsibility. He struggles with feelings of failure, particularly regarding his inability to protect his home and his sister, but channels this frustration into a fierce determination to fight. As the narrative progresses, he moves from being a bewildered newcomer to a resolute leader, eventually even undertaking a mission to rescue a former enemy, Reoko Banryū. His goal evolves from simply surviving to actively seeking the hidden truths behind the Buranki and ending the tyranny that grips post-apocalyptic Japan.

In terms of notable abilities, Azuma's primary power is his Bubuki, Oubu's Heart. This power does not grant him independent offensive capabilities in the way other Bubuki might, but it is the most crucial component for forming Oubu. He activates the fusion and controls the giant Buranki in battle. As a Heart-user, he also exhibits a form of regeneration, as he and his Buranki are able to recover from severe damage. His physical appearance, as noted in character profiles, includes blue hair and gray eyes, marking him as a distinct figure in the series' cast of characters.