Kash Gyustan, known as Hellstinger, emerged from a wealthy British family as the eldest son of a famous classical composer. Subjected to demanding violin training under his father's supervision from age six, mastering Johann Strauss, the oppressive environment drove him to escape his predetermined fate. A year before the main events, a mysterious man radiating a fiery aura appeared to him during a family gathering at their estate. This figure gifted Kash a Kaizer Stone, instructing him to find its true owner and promising it would rebirth his life. Kash accepted the stone and vanished from England, severing his past ties.

By seventeen, Kash reinvented himself in Neo Tokyo as the rockstar Hellstinger, embracing a heavy metal-inspired persona symbolizing liberation from aristocracy. This transformation granted him supernatural abilities linked to the Kaizer Stone. He adopted a flamboyant, outgoing demeanor, frequently inserting random English words like "baby" or "stage" into Japanese conversation, and relished attention as a self-proclaimed womanizer.

In the OVA, Kash's role expanded. He attended meetings with the antagonist Ohga, displaying disinterest in Ohga's schemes. He later confronted the protagonist Isato Kaiza, revealing the Kaizer Stones' origin: fragments of Ohga's power, born from his blood and inherently evil, though their use depended on the wielder's will. Kash recounted his own failed challenge to Ohga as Hellstinger, where he witnessed Ohga's vision of global destruction. This revelation strengthened Isato's resolve to confront Ohga.

Kash fought against Ohga's forces, including the Platonic Twins Ryo and Suzu Asahina. During a skirmish where Isato accidentally killed Ryo, Kash intervened to shield Isato. He later joined Isato and Kyosuke Shigure in storming Ohga's stronghold, battling violet energy entities summoned by Ohga, showcasing his shift from self-interest to collaborative resistance.

Hellstinger's powers centered on his guitar, wielded as a physical weapon. Key abilities involved generating a fiery aura by playing the guitar and executing the "Lyneck Stinger," throwing it like a boomerang to attack opponents. He also demonstrated flight as a winged humanoid. His fighting style blended aerial agility with projectile assaults, emphasizing his theatrical persona.

Kash's ending depicted him saving Suzu Asahina, who idolized him. This act inadvertently made him an enemy of her twin brother Ryo, reinforcing his role as a charismatic, morally ambiguous figure balancing destructive potential with sporadic heroism.