Kei is the real-world identity of the Dragon, a mysterious and feared avatar in the virtual world U, and he serves as a central figure in the story of Belle. In reality, he is a third-year middle school student, roughly fourteen or fifteen years old, living in Tokyo with his abusive father and his younger brother, Tomo. His home life is marked by physical and emotional violence, and he carries the burden of shielding his little brother from the worst of their father's behavior. This trauma shapes nearly every aspect of his existence, leaving him guarded, withdrawn, and deeply distrustful of others.

Inside U, Kei manifests as the Dragon, an imposing and scarred creature with an aggressive reputation. His avatar is covered in markings that resemble bruises, visually echoing the harm he endures in the physical world. The Dragon is notorious for disrupting the harmony of U through violent outbursts, which draws the attention of the Justices, a vigilante group led by Justin. The Justices consider him a threat to the order of the virtual society and relentlessly hunt him, convinced that he must be exposed and punished. For Kei, U functions as both an escape from his painful reality and a mirror of his inner suffering, since even in this imagined space he is isolated and attacked.

His motivations are rooted in protection and survival. He endures abuse largely to keep his younger brother safe, and his love for Tomo is the purest expression of his character. Beneath his fierce and defensive exterior lies a desperate need for help and acceptance, though he does not know how to ask for it. His anger in U is less about genuine malice and more about lashing out from a place of profound hurt.

The Dragon's role in the story becomes clear through his connection with Belle, the celebrated singer of U who is secretly the shy high school student Suzu Naito. Their first encounter is chaotic, with the Dragon crashing into one of her concerts and spreading panic, but Belle chooses to reach out to him instead of fearing him. She gradually uncovers his vulnerability and, with the help of her friends, discovers that the Dragon is an abused boy in the real world. Her persistent compassion allows Kei to lower his defenses, and he eventually lets her into his private digital space, revealing the pain he has kept hidden.

His relationship with Belle transforms both of them. She becomes the first person to show him genuine kindness in U, and their bond moves from confrontation to mutual healing. While Belle helps Kei feel seen and understood, his suffering also gives her the courage to face her own grief and reclaim her voice.

Kei's development culminates in an act of bravery that echoes the courage Belle demonstrates in revealing her true identity. Watching her risk everything to help him inspires him to stand up to his father in the real world, finally refusing to accept the abuse. His arc moves from silent endurance to active resistance, showing that accepting help and finding the strength to protect oneself are forms of healing.

In terms of abilities, the Dragon is one of the most powerful and feared presences in U. He commands destructive energy and can shatter the structures of the virtual world with raw force, which is what marks him as a danger in the eyes of the Justices. Yet his abilities are double-edged, representing both his untamed strength and the monstrous self-image he has developed as a result of his trauma. His physical power in U contrasts sharply with his powerlessness in the real world, emphasizing the gap between how he appears and how he truly lives.