Chobee Aza, also known as the Bandit King of Iyo, is a major supporting character whose life is defined by a fierce and unwavering devotion to his younger brother, Toma. Before the events on the island, Chobei was a notorious criminal leader, having amassed a long list of violent offenses that ultimately led to his death sentence. His descent into a life of crime began in childhood. He and Toma were born into a family of samurai retainers, but after their lord was forced to commit seppuku, the family fell from grace. Left as ronin, they sank into extreme poverty, which led to the death of their mother from illness and the eventual execution of their father for his involvement in a revenge plot. Orphaned and destitute, the two brothers were taken in by bandits, but Chobei's natural ferocity and intelligence allowed him to quickly seize control, rising to become their young leader.

On the surface, Chobei appears to be a purely barbaric and sadistic individual, often choosing violence as his first and only solution. He is arrogant, refuses to follow rules set by others, and declares himself to be his own master and god. He takes a cruel pleasure in the defeat of his enemies, a trait that remains with him throughout the story. However, this brutal exterior masks a sharp, calculating, and rational mind. While he may act like a brute, he is highly adaptable and capable of making shrewd, pragmatic decisions in life-or-death situations. His one and only absolute principle is the protection of his younger brother, Toma. Everything Chobei does, from his acts of extreme violence to his most strategic deceptions, is ultimately in service of ensuring both of their survivals.

Chobei’s role in the story is initially that of a chaotic and powerful wildcard. He and Toma are assigned to the expedition to find the Elixir of Life on the sinister island of Kotaku in exchange for a pardon from the shogunate. Once on the island, his raw strength makes him a formidable force against the island's monstrous flora and fauna. However, his real evolution begins when he and Toma face the island's godlike masters, the Tensen. After a crushing defeat, where he is utterly outmatched, Chobei makes a characteristically pragmatic choice: he decides to betray the human expedition and ally himself with the Tensen leader, Rien. He performs the required bochu jutsu, understanding that cooperation is the only immediate path to survival and to being reunited with his brother.

This decision sets the stage for his most significant transformation. After the Tensen gift him with the power of a Waitanhua flower, Chobei undergoes a dramatic physical change. His skin darkens, his eyes turn white, and he gains the ability to manipulate plant-like vines that sprout from his body. His key relationships are the core of his character. His bond with Toma is the story's most significant constant; they are a perfect and inseparable unit, with Toma often acting as the calmer strategist to Chobei's direct aggression. He also forms temporary and transactional alliances, such as with the executioner Fuchi and the warrior Gantetsusai, showing his ability to work with others when their goals align. His rivalry with the protagonist, Gabimaru, is another pivotal relationship, as their clash represents a battle between two skilled killers who have lost their sense of self. Ultimately, Chobei's development is less about moral awakening and more about a relentless and static focus on his goal. Unlike many other characters who seek redemption or change, Chobei remains fundamentally who he is from beginning to end: a bandit king whose sole unwavering purpose is to survive alongside his brother, even returning to that role after the island's final events.

Chobei possesses formidable abilities that make him a top-tier combatant. Even before obtaining supernatural powers, he was an exceptionally strong and durable human, capable of breaking a man's neck while his arms were bound. His true strength comes from his mastery of Tao, the fundamental energy of life. His Tao is aligned with the Metal attribute. After receiving the Waitanhua, his abilities are superhumanly enhanced. He gains an incredible regenerative healing factor that allows him to recover from otherwise fatal wounds, including a broken neck and severe bodily trauma. This form also grants him enhanced speed, enough to react to supersonic shockwave attacks, and increased strength that lets him scale far above other powerful characters. His signature technique involves freely controlling powerful, prehensile vines from his body to attack enemies from all angles, wearing the defeated Doshi’s robes after his initial transformation. His most notable tactic is his cold, analytical mind, which he uses to assess weaknesses and manipulate situations, such as storing bottles of Water Tao in his stomach for a future fight.