Heavy Grenadier is a formidable mercenary warrior who appears on the planet El Alamein, a former human battlefield now under the control of mechanical beings. He is described as a rude and untamed soldier, a man who fires first and asks questions later, making him an unpredictable and dangerous adversary. His imposing physical presence is matched by a personality hardened by a lifetime of conflict, leaving him seemingly suited only for warfare.

Initially, Heavy Grenadier's primary motivation is to continue his lonely war against the mechanical beings he believes conquered his world. His combat experience spans numerous stellar battlefields, and he carries the guilt of having helped devastate El Alamein himself during prior military actions. This sense of personal responsibility drives his most defining trait: a fierce, self-imposed duty to protect a group of war orphans, including the son of a fallen comrade-in-arms. He remains on El Alamein not for payment, but to fulfill a promise to their deceased fathers, protecting them from the very forces he once fought alongside.

Heavy Grenadier's role in the story begins when the battleship Karyu, captained by Warrius Zero, lands on El Alamein for repairs. Mistaking the Karyu for an enemy mechanical ship, Grenadier attacks it immediately. Impressed by the mercenary's courage and skill, Captain Zero seeks him out not as an enemy, but as a potential ally in his mission to hunt down the space pirate Captain Harlock. Grenadier is intrigued by the challenge of hunting such a famous pirate, but his sense of honor compels him to refuse, prioritizing his ongoing mission to shield the orphans from harm.

The relationship between Heavy Grenadier and Warrius Zero evolves from that of adversaries to trusted shipmates. Their turning point occurs when Luto, the rebellious eldest orphan, betrays Grenadier by revealing the location of their hideout to the mechanical beings in exchange for false promises of safety. When the base is bombarded, Grenadier fights fiercely to save the children but is overwhelmed and nearly killed. Zero arrives just in time to rescue him and provide the firepower needed to destroy the enemy bombers, saving the remaining children. After this event, Grenadier recognizes that Luto's actions, born of a desperate desire to end the cycle of violence, signify that the orphans no longer need his protection. Accepting this, he agrees to join Zero's crew on the Karyu, leaving behind a bag of seeds for the children to replant flowers on their scarred world.

This event marks the beginning of Heavy Grenadier's character development. He transitions from a lone, wild warrior fighting a personal war to a loyal member of an organized crew. He demonstrates a growing trust in Captain Zero's judgment and begins to socialize, albeit in his own rough manner, with the other officers aboard the Karyu. His professional bond with Zero becomes one of mutual respect as he aligns his formidable skills with the captain's leadership. Following his recruitment, he plays a key role in the ongoing search for Captain Harlock, suggesting they travel to the frontier planet of Heavy Meldar and the lawless city of Gun Frontier to find information, as it is a hub for bounty hunters.

In terms of notable abilities, Heavy Grenadier is a devastating direct combatant who leverages his immense battlefield presence and experience. He is proficient with heavy artillery and a long-range targeting scope, which he uses to engage enemy ships and bombers. He is also shown to be a capable field operative, leading reconnaissance missions on hostile planets. Above all, his true strength lies in his tenacity, courage, and unwavering sense of duty, fighting to the point of near-death to protect those under his charge.