Hajime Ryuudou, 23, is the eldest of four brothers who shouldered guardianship of his younger siblings after the deaths of their parents and grandfather. He formerly taught history at Kyowa College until his uncle, Seichiro Toba, caused his dismissal. Hajime possesses a generally calm and mild-mannered demeanor, though fierce protectiveness surfaces when his family is threatened. His exceptional physical strength and combat skills make him the most formidable fighter among his brothers. As the reincarnation of the Blue Dragon King of the East, he wields dominion over gravitational forces when transformed into his dragon form.

Hajime maintains distinct bonds with each brother: mutual respect and companionship define his relationship with Tsuzuku (the second brother), mentorship guides his interactions with Owaru (the third brother), and heightened protectiveness characterizes his care for Amaru (the youngest brother), whose vulnerability and developing powers require his guidance. He shares a close familial bond with his cousin Matsuri Toba, who assists in managing their household; all four brothers are deeply protective of her. His antagonistic relationship with Seichiro Toba stems from the uncle's greed-driven manipulation, which cost Hajime his career and continues to threaten the family's stability, cementing Hajime's role as the family's primary defender against external threats.