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Ichirouhiko is a primary antagonist in the story, serving as a dark mirror to the protagonist, Kyuta. He is the adoptive older son of Iōzen, a respected master swordsman in the Beast Kingdom, and the brother of Iōzen's biological son, Jirōmaru.

Raised from infancy in the Beast Kingdom, Ichirouhiko was a human baby abandoned in a human city and discovered by Iōzen. Despite a prohibition against humans in the beast realm, Iōzen adopted the child and raised him as his own, leading Ichirouhiko to believe he was a beast. As he grew, he became troubled by his appearance, noticing he lacked the snout and tusks of his boar-like father. This self-awareness fostered a deep sense of shame and self-loathing that he could never fully reconcile. In the Beast Kingdom, humans are forbidden because the negative emotions in their hearts can manifest as a spiritual "emptiness," granting supernatural power but also leading to hostility and madness. Ichirouhiko's self-hatred quietly fed this darkness inside him throughout his youth.

In terms of personality, Ichirouhiko initially appears as a composed and dutiful son who admires his father as an idol. He is protective of his younger brother, Jirōmaru, and in their childhood, he acts as the more responsible one, chastising Jirōmaru for bullying the newly arrived human, Kyuta, though he does so in a belittling manner. As he matures, his demeanor curdles into bitterness, arrogance, and intense jealousy. He is driven by a hatred for Kumatetsu, Iōzen's rival, and for Kyuta, who becomes Kumatetsu's successful apprentice. His envy stems from Kyuta's excellence and acceptance, qualities Ichirouhiko feels he can never achieve as a human in a beast's world. This jealousy ultimately consumes him, leading him to the edge of insanity.

Ichirohiko's primary motivation is a desperate and tragic need to emulate his father and be worthy in his eyes. He trains rigorously in martial arts to please Iōzen and uphold his family's honor. However, his inability to become a true beast, coupled with Kyuta's rise, causes his respect for his father to curdle into a fanatical need to see Iōzen succeed at any cost. The thought of Kumatetsu, whom he considers inferior, defeating his father is unimaginable to him, and this hatred is the direct fuel for his corruption.

His role in the story is to act as the central antagonist whose downfall forces the climax of the film. He is Iōzen's son, which places him in direct opposition to Kumatetsu and Kyuta, the apprentices of the rival dojo. While he initially ignores Kyuta, his hidden resentment grows for eight years, culminating in his actions that trigger the final conflict. After Kumatetsu defeats Iōzen to become the new Lord of the Beasts, a maddened Ichirouhiko uses telekinesis to impale Kumatetsu with his father's own sword. Following this act, he flees to the human world, where his emptiness fully manifests into a destructive monster.

Key relationships define his arc. With his father, Iōzen, Ichirouhiko shares a bond of deep admiration and desperate striving for approval. His relationship with his brother, Jirōmaru, is initially loving, but as Ichirouhiko's bitterness grows, he becomes dismissive and even violent, shoving Jirōmaru aside for suggesting their father might lose. Ichirouhiko's relationship with Kyuta is the most crucial; they are narrative foils. Both are human boys raised by beast fathers, but where Kyuta finds acceptance and love, Ichirouhiko finds only shame and envy. While Ichirouhiko defends a young Kyuta from his brother's bullying, his condescending attitude plants the seeds for later antagonism. As adults, he despises Kyuta for being the person who supports Kumatetsu.

Ichirouhiko undergoes a significant development, transforming from a respected noble's son into a villain consumed by darkness, before being saved in the resolution. After being defeated by Kyuta, who uses Kumatetsu's reincarnated power to purge the emptiness from his heart, Ichirouhiko awakens with no memory of his rampage. He returns to a much more level-headed state, and he notices a bracelet left on his wrist by Kyuta as a gesture of kindness and understanding. He is last seen waking up in his bed at home with his family standing beside him, implying a path toward peace and self-acceptance.

In terms of notable abilities, Ichirouhiko wields the power of the "emptiness," the dark manifestation of his negative emotions. This grants him low-level telekinesis, which he eventually uses to levitate and manipulate swords with deadly force. As he is fully consumed by his hatred, his power grows dramatically, allowing him to shapeshift. His most spectacular ability is transforming into a gigantic, shimmering white whale with wild boar tusks, a form inspired by the novel Moby Dick. In this state, he unleashes massive destruction on the human city of Shibuya before his final confrontation.
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