Seiryū belongs to the Kitoralsus species, initially serving as a primary antagonist against humanity. His species inhabited Earth before humans, and after part of their clan departed for space, he remained among the Agents under Iza's command. Their mission centered on reclaiming Earth from humans for their species' survival.

Seiryū and fellow Agents employed Black Particles to control objects and animals, deploying them against human settlements. These actions provoked confrontations with Shinkalion teams protecting humanity. Through interactions with humans—especially Hayato Hayasugi—Seiryū evolved significantly. He grappled with understanding human behavior and modern lifestyles, starkly different from his species' past. Gradually, he formed genuine bonds with the Shinkalion team, transitioning from adversary to protagonist and viewing them as equals and friends, mirroring teammate Genbu's arc.

Seiryū shares deep familial ties with fellow Kitoralsus Genbu, Suzaku, and Byakko. Byakko's petrification and Genbu's death triggered profound sorrow and rage in him. Hayato's influence sparked Seiryū's fascination with shinkansen and railways. Exposure to human culture also cultivated his pronounced fondness for sweets, beginning with cake and expanding to all confections.