Kirōmaru is a prominent character from the anime, serving as the military leader of the Giant Hornet colony of Monster Rats. He is introduced as a counterpart to the more devious Squealer, embodying a sense of nobility, strength, and honor that sets him apart from other creatures of his kind. Physically, he is depicted as large and regal, with a commanding presence that resembles a lion more than a typical rat, which immediately establishes him as a figure of respect and authority.

His personality is defined by a strong sense of duty, loyalty, and traditional values. As a leader, he is rational and makes detached decisions, often prioritizing the safety and preservation of his clan above all else. It is noted that he cares deeply for his people, and would withdraw from a dangerous situation to protect a single one of his men rather than risk unnecessary casualties. He is wise, loves to fight, and possesses a business-like demeanor, being blunt and down-to-earth in his interactions. Unlike Squealer, who schemes to liberate his race through rebellion, Kirōmaru is content to serve the humans, believing that cooperation is the best path to ensuring his clan's survival. He values order and tradition, and he becomes frustrated when others allow their emotions to cloud their judgment, as seen when he reprimands Saki for becoming discouraged instead of thinking of a plan.

Kirōmaru's role in the story is complex, as he is both a loyal ally to the human protagonists and a tragic figure caught in the oppressive world order. He initially allies with the humans Saki and Satoru, helping them defeat their pursuers and demonstrating his capabilities as a formidable warrior and strategist. His primary motivation is the preservation of his clan, the Giant Hornets. While he does attempt to obtain nuclear weapons at one point, suggesting a desire for power, he never pursues the brutal path of liberation that Squealer does, instead remaining a faithful if not subordinate partner to the human society. He respects strength and rationality, and his word carries significant weight among the Monster Rats. In the final act of the story, Kirōmaru makes the ultimate sacrifice. During Squealer's rebellion, where a child raised as a Fiend threatens the human village, Kirōmaru purposefully provokes the child to kill him. This act triggers the child’s Death Feedback, the psychological block that kills a psychic human who takes another human life, thereby ending the rebellion and saving the human settlement.

His key relationships are primarily with the human characters Saki and Satoru, whom he serves alongside with a sense of professional respect. He stands in stark contrast to Squealer, representing the alternative path for Monster Rats: honorable cooperation versus violent revolution. Throughout the series, his development is subtle but poignant. He begins as a proud general and ends as a martyr, sacrificing himself not just for the humans, but for the stability of the world, hoping to secure a future for his people. In terms of notable abilities, Kirōmaru is a highly skilled military strategist and a powerful physical combatant. He is capable of commanding armies and using advanced weaponry. He possesses a specific barrier ability, using his Cantus to create reflective surfaces that are crucial in battles against other psychic users. His most significant act, however, is not a power but a strategic sacrifice, using his own death to weaponize the enemy's psychological limitations.