Chobei, also known as the Bandit King, is a major supporting character in the narrative. Before the events on the island, he was the feared leader of a band of thieves in the Iyo province, accumulating a long list of crimes including murder, theft, and assault. His past, however, is rooted in tragedy. Chobei and his younger brother Toma were born as vassals to a lord in the Ako Domain. After their lord was forced to commit seppuku, the family fell into poverty, leading to their mother's death from illness and their father's execution. Left to fend for themselves, the two brothers were captured by thieves, but Chobei eventually took over the group, becoming their leader. It was during this time that he inflicted a notable scar and the loss of his own eye in an act of protection for Toma. After being arrested and sentenced to death, Chobei was scouted for a mission: to travel to a mystical island in search of the Elixir of Life in exchange for a full pardon from the shogun.

Chobei's personality is defined by a fierce and often brutal pragmatism. He is a ruthless criminal who has chosen to live by his own rules and does not hesitate to resort to violence, displaying a sadistic streak in combat. He holds grudges against those who defeat him and prefers direct, forceful solutions over negotiation. Despite this barbaric exterior, Chobei is a rational thinker who can quickly assess dangerous situations and decide on the most effective course of action. His most defining trait, however, is his overwhelming protectiveness over his younger brother, Toma. Everything he does, every risk he takes, is ultimately motivated by a desire to ensure Toma's safety. He is entirely willing to take on the role of the villain to shield his brother from harm.

Within the story, Chobei is one of the most powerful death row convicts sent to compete for the Elixir of Life. Unlike the protagonist Gabimaru, who struggles with his humanity, Chobei embraces his monstrous side to survive. His role initially places him as an antagonist to Gabimaru, driven by a personal grudge and a desire to prove his strength. Chobei is also an exceptionally quick learner in the art of Tao, the fundamental energy of life on the island. His ability to adapt is demonstrated when he and Toma are thrown into a pit of strange flowers; rather than being consumed, Chobei's body undergoes a forced evolution. He becomes immortal, gaining the ability to regenerate from any wound, including a torn throat, broken limbs, and even being burned alive. This regeneration, however, comes with a risk: overuse allows the plant-like vines within him to take over, transforming him into a monstrous, demon-like entity with tentacles, blackened skin, and claws. In this state, he loses control and attacks indiscriminately.

His relationship with Toma is the emotional core of his character. Toma often acts as the voice of reason, worrying about his brother's well-being and the cost of his transformations. While Chobei always believed he was protecting Toma by keeping him out of fights, Toma eventually comes to resent this, fearing he is seen as a burden. This dynamic pushes Toma to seek his own strength, but Chobei's primary drive remains their reunion and survival. During his journey, Chobei allies with others when necessary, using his cunning to guide them against more powerful enemies like the Tensen, the island's lord-like beings. In a key development, Chobei is captured by the Tensen leader, Rien. Rather than fight a losing battle, Chobei adapts again, cooperating with Rien and even undergoing a process of Tao regeneration to gain more power and an opportunity to see his brother again. By the end of his journey, Chobei endures a profound physical change, being reborn into a smaller, toddler-like state, yet his indomitable will and ferocity remain intact.