Biba Amatori is a central figure in his story, initially presented as a heroic savior before being revealed as its primary antagonist. He is a young man of twenty-two years, recognized for his striking appearance characterized by long, pink hair and sharp golden eyes. Known by the public as the Liberator, he commands an elite and highly independent anti-Kabane unit called the Hunters. Despite being the eldest son of the shogun, he has been officially disowned, a fact that fuels his hidden vendetta against the ruling government.

Biba's personality is a complex mask of charisma and cold cruelty. To the masses, he is a noble and beloved hero who fearlessly battles the Kabane, offering hope and protection. He is articulate and capable of inspiring fierce loyalty among his followers. However, this public image conceals a deeply cynical and ruthless interior. He adheres to a strict, survival-of-the-fittest philosophy, believing unequivocally that the strong survive and the weak deserve to die. He has no qualms about smiling while taking a life or callously discarding those he deems weak or useless. His actions are driven by a profound desire for revenge, and he is willing to sacrifice anyone, including his own soldiers, to achieve his goals.

Biba's motivations are rooted in a traumatic betrayal by his father, the shogun. When Biba was twelve years old, he was sent to command a massive army to fight the Kabane. His father deliberately cut off their supply lines, leading to a catastrophic defeat and the deaths of hundreds of thousands of his men. This act of abandonment shattered Biba, replacing any filial affection with an all-consuming hatred. His primary motivation is to exact revenge on his father and the entire shogunate system that enabled the betrayal. He rationalizes his destructive plan as a form of liberation, aiming to tear down the station walls that confine humanity, which he views as cages.

Within the narrative, Biba serves as the main antagonist. His role begins when his train, the Kokujou, arrives at a station where the protagonists are located. He is initially hailed as a hero, but his true nature is gradually uncovered by Ikoma, a self-made Kabaneri who distrusts his philosophy. Biba masterfully manipulates situations and people to his advantage. He secretly transports a horde of Kabane inside his own train, using them as a weapon to massacre entire stations under the guise of a rescue operation. His ultimate goal is to reach the capital, Kongoukaku, to personally confront and destroy his father.

Biba's most significant relationship is with the young girl named Mumei, whom he saved as a child. After saving her, he forced her to kill her mother's murderer, instilled in her his ruthless worldview, gave her the name Mumei meaning nameless, and subjected her to the experiment that turned her into a Kabaneri. She idolizes him, calling him brother, but he views her purely as a tool, praising her only to manipulate her loyalty. His relationship with the protagonist Ikoma is one of contrast and conflict. Both are Kabaneri who reject the established order, but while Ikoma fights to protect others, Biba fights only for his own vengeful destruction. He is also shown to have other Kabaneri followers, such as Horobi, whom he also treats as disposable assets.

Biba himself is a Kabaneri, possessing the enhanced strength and durability that comes with the condition. He is an exceptionally skilled and calculating strategist, having built the Hunters into a formidable military force equipped with advanced weaponry like mortars and motorcycles. In combat, he wields a distinctive gun-sword coated in Kabane heart metal, demonstrating incredible proficiency. He is also revealed to be the mastermind behind the research on Kabaneri, possessing the knowledge and resources to artificially create them and even to create monstrous fusion beasts known as the Black Smoke.

Biba's development is less about growth and more about the single-minded pursuit of his revenge. He is a tragic figure whose humanity was consumed by his father's betrayal. After finally confronting and killing his father, he appears hollow and lost, having achieved his only goal. In his final moments, he seeks a final, meaningful fight against Ikoma, which he ultimately loses. In a final act of ambiguous intent, he injects a white serum into Ikoma to save him from his own rampage before being killed by Mumei, who finally breaks free from his control.