Born Yamanouchi Naotoki into a low-ranking Satsuma samurai family under Shimazu dominance, limited prospects drove him to seek recognition as a mercenary. He joined the nearly collapsed Kōno clan navy fighting the Mouri in the Seto Inland Sea, hoping distinction would come easier among the powerless. Following a crushing naval defeat where he faced overwhelming odds, Naotoki succumbed to despair and self-recrimination, cursing his naivety. This intense remorse drew the demon lord Arago, who exploited his vulnerability.

Arago corrupted Naotoki, twisting his inherent virtue of obedience ("Tei")—centered on caring for the aged and humble—into absolute loyalty solely to the demon lord. Brainwashed to oppose all others, he transformed into the Dark Warlord Naaza, donning the Doku armor. Linked to the healing Buddha Yakushi Nyorai yet imbued with poison's essence, the armor paradoxically held latent potential for healing and medicine. Its design, inspired by the Indian Naga, featured snake motifs on his mask and weapons. Traces of his original compassionate nature persisted beneath his antagonistic role despite the corruption.

As one of Arago's four Dark Warlords representing Autumn, Naaza wielded the Jaga Ken: six segmented blades combining into an extended whip-like form for his signature Rokusetsu Orochi attack. He employed poison to inflict paralysis, blindness, or possession, demonstrated telekinetic control over his swords, and possessed rapid regenerative abilities, likening himself to a snake regrowing its tail. His battles frequently targeted Shin Mouri, the Ronin Warrior of Water, leveraging his history of naval combat.

Naaza met his end due to enemy action during the conflict against the Ronin Warriors, concluding his character arc with his demise at the end of the first major story arc within the main series narrative.