Kai Etō, a fledgling investigator in the Narcotics Control Department, works alongside a stoic partner to dismantle drug networks. Orphaned in childhood by Sawaki—a high-ranking Ryuugen syndicate member—his trauma birthed a volatile alter ego that surfaces during emotional distress, operating without restraint to neutralize threats. This persona, forged as a psychological shield during his parents’ murder, was later subdued via hypnotic therapy administered by his partner’s father, erasing memories of the crime and an enigmatic drug tied to the incident.

Adopted by his aunt and renamed Etō, he balances a compassionate nature with fierce determination, prioritizing visceral instincts over methodical planning when justice is at stake. His unwavering loyalty to his partner manifests in life-risking devotion, contrasting her clinical professionalism, though their dynamic thrives on unspoken mutual guardianship.

The repressed alter ego reemerges under extreme duress—especially threats to loved ones or echoes of his past—leaving no recollection of its actions. His history remains entangled with Ryuugen: Sawaki, his parents’ killer, once spared subordinate Akaha, who later died protecting him. Physically distinctive with cropped red hair and crimson eyes, Kai’s presence commands attention.

Animated adaptations subtly deepen his partnership’s nuances without altering his core identity. Suppressed memories and the dormant alter ego fuel latent tensions in his investigations, as recurring clashes with Ryuugen unravel threads connecting him to a past he cannot fully remember.