Thirteen-year-old Amaru Ryuudou is the youngest of four brothers, reincarnations of divine Dragon Kings. As the Black Dragon King of the North, he commands the elements of water and lightning in his dragon form. His youth makes these powers potent yet unpredictable, manifesting more strongly but controlled less easily than his older brothers. These abilities prove vital to the siblings, especially in human form.

Amaru possesses a gentle, caring nature and moments of surprising maturity, contrasting his energetic third brother, Owaru. Despite this maturity, he idolizes Owaru and actively joins his pursuits. He seeks guidance from his eldest brothers, Hajime and Tsuzuku, and rarely starts fights, though his combat skills emerge over time. He experiences prophetic dragon dreams, interpreted by Hajime as warnings or insights into their challenges.

Amaru shares a deep bond with his brothers. Protective Hajime acts as his guardian, patient Tsuzuku offers support, and Owaru serves as a role model he follows enthusiastically. All four brothers steadfastly protect their cousin, Matsuri Toba, whom Amaru views as a maternal figure. Their unity faces repeated tests from adversaries like the "Four Sisters" and the "Bulls," shadowy organizations seeking to harness the dragons' powers for global domination. These conflicts force the brothers to flee while confronting their identities and destinies.

Amaru's development focuses on balancing his human life with his dragon heritage amid rising threats. Initially perceived as vulnerable, he gradually reveals resilience and latent power, particularly during critical moments when his elemental control becomes pivotal. His journey mirrors the brothers' broader struggle to protect their family and thwart those attempting to manipulate their ancient legacy.