Kei is a third-year middle school student living in Kawasaki, Kanagawa Prefecture, with his younger brother Tomo and their abusive father. After their mother's death, both brothers endure ongoing physical and verbal abuse. Kei frequently shields Tomo from their father's violence, resulting in extensive bruising across his back.

Within the virtual world "U," Kei becomes the Dragon, a towering draconic avatar distinguished by white horns, sharp claws, and a tattered red cape. This cape visually translates Kei's real-world bruises into its design, symbolizing his hidden trauma. The Dragon possesses unmatched combat strength in U, fueled by Kei's pent-up rage and determination to protect Tomo. This power allows him to dominate opponents, despite being pursued by the vigilante group Justices led by Justin.

The Dragon inhabits a concealed castle in U, safeguarded by AI guardians that redirect outsiders. This castle serves as a sanctuary for Kei and Tomo, who appears there as the silent Angel avatar, reflecting his timid real-world demeanor. Though the Dragon exhibits outwardly aggressive behavior, serving as a defense mechanism against mistrust, he shows gentleness toward children in U, who view him as a hero. His aggression contrasts with his reluctance toward physical or emotional intimacy, which gradually eases only through Belle's persistent kindness.

Kei's development begins when Belle locates his castle and offers aid, though he initially rejects her. After the Justices destroy his sanctuary, Belle uncovers his real identity and reaches out. Kei distrusts her assurances, interpreting them as hollow gestures common in his experience. To prove her sincerity, Belle publicly unveils her real identity in U and sings openly. This compels Kei to trust her, but his father severs their internet connection as they attempt to share their location.

Using environmental clues from a live stream, friends deduce Kei's hometown. Suzu travels there and intervenes during a confrontation with their father. She physically shields the brothers, enduring injury until Kei's father retreats, confronted by her defiance. This resolves Kei's immediate danger and concludes his arc as he accepts external support. Post-rescue, Kei's appearance changes slightly, his hair no longer obscuring one eye.

Thematically, Kei embodies the "wounded child" archetype, using U to express suppressed emotions. His Dragon persona channels pain from abuse into strength, a dynamic reflecting Director Mamoru Hosoda's commentary on societal issues often overlooked in animation. Hosoda's research with child protection agencies informed Kei's portrayal, highlighting real-world challenges like intergenerational abuse and systemic barriers in child welfare. Kei's mother remains a poignant absence; a shattered portrait of her in his U castle hints at unresolved grief potentially fueling his aggression.