Reijiro Araki, adoptive father of Yukiya Araki, assumed guardianship of the infant after a mysterious couple entrusted him to Reijiro and his wife Yukine. Once a Hand Shaker operating under the team name Chain, he partnered with Yukine—codenamed Break—to battle within the Ziggurat, an alternate dimension where combatants vie for wishes granted by a deity. Post-conflict, the couple abandoned their warrior roles to build a peaceful life, shielding Yukiya from their past by forbidding physical contact to suppress his latent Hand Shaker abilities.

Hotheaded yet fiercely protective, Reijiro channels his intensity into a career as a music engineer, nurturing a shared passion for house music with Yukiya. His attempts to preserve normalcy unravel when Yukiya stumbles into Hand Shaker conflicts, compelling Reijiro to revisit his history. The couple later uncovers Yukiya’s origins: his biological parents, Nagaoka and Mayumi, conceived him via a Ziggurat wish during their own Hand Shaker trials.

Reijiro’s journey pivots on reconciling his desire to protect Yukiya with the inevitability of the youth’s destiny. His experience as Break fuels both his grasp of the dangers and friction with Yukiya over autonomy versus duty. The adoption’s catalyst—a confrontation with the enigmatic couple—remains shrouded, yet Reijiro’s resolve solidifies into supporting Yukiya’s path, prioritizing familial unity over the weight of their inherited legacy.