Kash Mizutani, alias Hellstinger, is the son of a renowned British classical music composer. Chafing under expectations to follow his father's path, he rebelled, seeking independence. A fateful encounter with a mysterious, flame-aura figure granted him a Kaizer Stone and spurred him to leave home. This led him to embrace rock and roll and enroll at the Bernard Institute.

Initially indifferent to the conflict surrounding Principal Shizuru Ozaki, Kash once held Ozaki in high regard but grew disillusioned with Ozaki's methods, creating a rift that distanced him from the cause. Concurrently, repeated interactions fostered growing sympathy and camaraderie with Isato Kaiza (Gowcaizer). While hesitant to openly oppose Ozaki at first, Kash ultimately chose to align decisively with Kaiza. He possessed confidential knowledge of Ozaki's hidden activities, keeping much of it secret even as his bond with Kaiza strengthened.

His journey traces a shift from a conflicted individual seeking freedom to a committed ally against Ozaki and the manifestation of negative human desires, Omni Exist. Kash crucially provided Isato Kaiza with the Caizer Stone, enabling Kaiza's transformation into Gowcaizer. Following Kaiza's accidental killing of Ryo Asahina in battle, Kash aided Kaiza in grappling with the emotional fallout and understanding the nature and responsibilities of the Caizer Stone, marking significant maturation for both.

In the final confrontation, Kash, fully embracing his role as Hellstinger, united with Gowcaizer and allies to combat Ozaki's forces and the existential threat of Omni Exist, demonstrating his complete commitment to protecting humanity.

As Hellstinger, he transforms into a winged humanoid form. His primary weapon is an electric guitar, wielded physically in combat rather than played musically. His fighting style incorporates moves like the "Wryneck Swinger," "Voltage Crack," "Voltage Explosion," and the powerful "Dance of Death".