The Black King is a central antagonist and the leader of the Ends in the narrative. His origins are shrouded in mystery, but strong implications within the story suggest he was once a figure who sought to save humanity, only to be rejected and killed by them. This traumatic betrayal is the cornerstone of his existence, transforming him from a potential savior into a being determined to see humanity destroyed. Speculation frequently points to him being a version of Jesus Christ, a theory supported by his past actions of performing miracles and his current state of profound disillusionment with mankind. He now leads a vast army of non-human races, including goblins, kobolds, dragons, and other beings that humanity considers monsters, uniting them under a single banner in the northern lands.

In personality, the Black King is a figure of stark contrasts, embodying both immense cruelty and surprising benevolence. He is a charismatic and calculating leader who commands absolute loyalty from his followers. His hatred for humans is deep and all-consuming, leading him to orchestrate massacres and show no mercy to his former species. However, towards his own non-human subjects, he is a compassionate and caring ruler. He treats the goblins, orcs, and other so-called monsters not as disposable tools but as his compatriots, healing their wounds, providing them with food, and showing them genuine respect. This duality makes him a uniquely complex villain; he is not a force of pure chaos but rather a revolutionary leader building a new world order for the oppressed, even if that new order requires the annihilation of the old one.

The Black King's primary motivation is not mere destruction but the systematic replacement of human civilization with one built by non-humans. He believes that humanity is a cruel, oppressive, and self-destructive force that has abandoned its ethics in favor of unchecked development. His goal is to create an eternal dark age under his control, a world without the fear of extinction or the contamination of the planet, by denying all unnecessary progress. To this end, he is not just a conqueror but a founder of civilizations. He teaches his army how to farm, creates a common written language based on Latin, and establishes a unified religion to forge a single cultural identity from a multitude of disparate monster races. In his own eyes, he is not ending the world but overseeing a necessary generational change, replacing a failed species with one that will live in a stable, controlled stasis.

Within the story, the Black King serves as the primary antagonist and the ultimate threat that the Drifters must oppose. His role is to act as a dark mirror to the protagonists. While the Drifters are historical figures brought to a fantasy world to save it, the Black King is an End, brought there to destroy it. His actions drive much of the central conflict, as his unified army of non-humans clashes with the human-led forces of the Orte Empire and the Drifters. He is the architect of a grand strategy that involves seizing air superiority with dragons, using airborne troops in tactics reminiscent of 20th-century warfare, and building a self-sustaining war machine that seems nearly unstoppable. His presence elevates the conflict from a simple war to a battle for the very concept of civilization and the future of the world's dominant species.

The Black King's key relationships further illuminate his character. His most notable subordinate is Grigori Rasputin, who serves as his right-hand man and is entrusted with crucial tasks like teaching the monsters the new common language and religion. He shares a deep bond of mutual respect with his army; he provides for them pathologically, and they worship him as their savior in return. He also has a significant, if adversarial, relationship with the Drifter Abe no Haruakira. Despite being enemies, the Black King engages with Haruakira as an intellectual equal, discussing his philosophy and showing a grudging respect for his rival's convictions even as he rejects them.

The character shows subtle but important signs of development and vulnerability. While he projects an image of a god-like being, he explicitly denies being immortal, stating there is only one God. This is supported by the fact that his body is slowly turning to salt, a deterioration he keeps secret from his followers to avoid causing panic. This physical decay suggests that his immense power comes at a price and that he is not an eternal, invincible force but a being who is, in a very real sense, dying. His entire project of building a self-sustaining civilization for monsters, teaching them to farm and create their own culture, is framed as a necessary step because he will not be around to provide for them forever.

The Black King possesses an ability centered on the manipulation of life itself, often referred to as cell multiplication or replication. This power allows him to duplicate any living cell, whether plant or animal. He uses this ability defensively to heal wounds, both his own and those of his soldiers, and to instantly replicate food, such as multiplying a single ear of wheat into a mountain of grain. Offensively, this power is terrifying; he can cause the cells of a living being to multiply uncontrollably, effectively giving them a fast-acting, terminal cancer. He demonstrated this by subduing a massive Bronze Dragon, forcing it into submission by filling its body with cancerous growths. Beyond his supernatural abilities, the Black King is a brilliant military strategist and an unparalleled civil engineer. His knowledge of tactics from beyond the fantasy world's medieval level, such as using dragons for paratrooper-style assaults, marks him as a multi-world expert and a nightmare for his opponents on any battlefield.