Kei, a fourteen-year-old boy, lives in Kawasaki, Kanagawa Prefecture with his father and younger brother Tomo. Their father subjects both to routine physical and emotional abuse since their mother's death. Kei consistently shields Tomo, enduring violence meant for his brother, resulting in visible bruises that mark his body. This trauma defines his life and drives him.

Within the virtual world "U," Kei becomes the towering avatar Dragon, possessing a draconic visage with sharp teeth, white horns, yellow eyes, and a vibrant blue mouth and nose. He wears a tattered red cape adorned with colorful markings that mirror his real-world bruises, a consequence of "U's" biometric link. The Dragon wields immense strength and aggressive combat tactics, unleashing swift claw attacks capable of shredding opponents' data, fueled by Kei's protective rage toward Tomo. Though notorious for disrupting virtual stability and hunted by the vigilante Justices, the Dragon commands fierce admiration from "U's" children, who see him as a hero.

The Dragon resides in a concealed "U" castle guarded by loyal AI entities that misdirect outsiders; they address him as "Master." This castle serves as a sanctuary for him and his sole friend, the child avatar Angel (Tomo's counterpart). While the Dragon displays a wary, defensive exterior—repelling touch and scrutiny—this masks a fundamentally gentle nature. His vulnerability surfaces only after Belle (Suzu Naito's avatar) earns his trust through persistent kindness, culminating in him accepting her physical comfort.

Kei's relationship with his father is characterized by apathy and resentment, devoid of respect or affection. His entire focus is protecting Tomo, a duty empowering his virtual persona. Kei deeply mourns his deceased mother; a shattered portrait of her rests amid debris within his "U" castle, suggesting his grief manifests as uncontrolled aggression. His real-world appearance contrasts sharply with his avatar: a slender teenager with a dark tan, light brown eyes, and black hair often brushed to conceal his left eye—a habit that changes after his father departs.

Kei's path shifts when Belle discovers his identity and witnesses his father's abuse via livestream. Initially distrustful of Belle's help, he is moved when Suzu publicly reveals her real identity within "U" and sings to affirm her sincerity. This act, coupled with Tomo's encouragement, persuades him to accept assistance. His father retaliates by severing their internet connection to prevent exposure, but Kei attempts to share their location. Unable to secure immediate official intervention, Suzu confronts the father directly in Kawasaki, causing him to flee. Kei later expresses gratitude to Suzu, implying tentative steps toward healing and trust.