Ichirōhiko is a central figure in the story, serving as its primary antagonist. He is introduced as the first son and apprentice of Iōzen, a noble and highly respected master swordsman in the Beast Kingdom. Ichirōhiko is a young man with a slender build, very pale skin, dark blue hair, and blue eyes. He is almost always seen wearing a distinctive, oversized boar-shaped hat, which serves to obscure his features and becomes a key symbol of his internal struggle.

His background is rooted in a foundational secret. As an abandoned human infant, he was discovered in an alleyway in Tokyo by the beast nobleman Iōzen. Despite a prohibition within the Beast Kingdom against allowing humans into their realm, Iōzen adopted the child and raised him as his own, alongside his younger biological son, Jirōmaru. Throughout his childhood, Ichirōhiko was led to believe he was a beast, training diligently in martial arts to emulate his father. However, as he matured, the physical reality became undeniable. Unlike his father and brother, he grew no boar-like snout or tusks, which filled him with a deep sense of shame and self-loathing.

This internal conflict is the core of Ichirōhiko's personality and motivations. He is consumed by jealousy and an overwhelming desire to be exactly like his idolized father, a state he can never achieve. These negative emotions manifest as a spiritual corruption known as the emptiness, a dark power unique to humans in the Beast Kingdom that grants supernatural abilities but drives the user to hostility and madness. His hatred eventually focuses on Kyuta, another human boy who becomes the apprentice of Iōzen’s rival, Kumatetsu. While Ichirōhiko outwardly dismisses Kyuta as weak in their youth, he grows to despise him for excelling as a martial artist and for supporting Kumatetsu, a position Ichirōhiko sees as an insult to his own father.

In the story's climax, the emptiness fully consumes him. After Kumatetsu defeats Iōzen in the decisive battle to become the new lord of the Beast Kingdom, the humiliated and enraged Ichirōhiko uses telekinesis to impale Kumatetsu with his father's own sword before fleeing into the human world. His role then shifts from a jealous son to a rampaging force of destruction. In the streets of Tokyo, he uses his supernatural powers to cause massive destruction, even transforming into a giant, monstrous white whale inspired by the novel Moby Dick. He is ultimately defeated not by brute force, but by Kyuta, who, with the help of the reincarnated spirit of Kumatetsu, purges the darkness from Ichirōhiko's heart.

His key relationships define his arc. He worships his adoptive father, Iōzen, and his entire identity is built on being a worthy successor to him. He has a complicated bond with his younger brother, Jirōmaru; while he is protective in his own way, his rage grows so intense that he eventually shoves Jirōmaru aside for questioning their father’s superiority. His relationship with Kyuta is one of a dark mirror. Kyuta represents who Ichirōhiko could have become. Both are human boys raised by beasts, but where Kyuta finds acceptance and love, Ichirōhiko finds only resentment and insecurity.

His most notable development is his redemption. After being defeated and cleansed of his emptiness, Ichirōhiko awakens as a much more level-headed young man with no memory of his rampage. He finds a simple string bracelet tied around his wrist, a charm left there by Kyuta as a gesture of empathy and kindness. Noticing this token of appreciation, he shows signs of genuine change, finally at peace with himself. As a human in a beast's world, his notable abilities include low-level telekinesis, shape-shifting, and immense strength fueled by the negative energy of the emptiness, eventually allowing him to transform into a colossal, whale-like monster.