Amaru Ryūdō, the youngest Ryūdō brother at thirteen, governs the Northern Oceans as the Black Dragon King, commanding water and lightning in dragon form. Though he may appear vulnerable initially, his considerable supernatural abilities prove vital to his brothers, especially while they remain human. Tied to his emotional state and youth, his control over these powers is less stable than his siblings'.

He displays a gentle, caring nature and occasional maturity beyond his years, even surpassing his third brother Owaru. Despite this, he looks up to Owaru and frequently joins his activities. Amaru relies on his older brothers Hajime and Tsuzuku for guidance and seldom starts confrontations. As events unfold, he gradually reveals more of his combat skills and capabilities.

Amaru experiences prophetic dreams concerning the dragons' legacy, interpreted by Hajime. These dreams foreshadow events and deepen his connection to their divine origins. Due to his age, his dragon powers manifest more readily, serving as both an asset and a vulnerability in conflicts.

Within the family, he receives protection from all three brothers: Hajime as primary guardian, Tsuzuku providing supportive oversight, and Owaru assuming an especially close guardian role. Together, they strive to maintain normalcy while countering global threats like the Four Sisters organization and their predecessors, the Bulls. Throughout these struggles, Amaru’s abilities evolve, reinforcing his essential role in their collective destiny.